Improve accessibility of forms elements within some "form-table" forms

[afercia]

Follow-up to #46899.

In #46899, an attribute role="presentation" was added to most of the form-table used in the admin pages. These tables contain forms and in most of the cases they are used just for layout purposes. The role="presentation" helps in removing the table semantics where appropriate.

Note: this should be considered a temporary improvement, pending decision on the proposal to entirely remove layout tables from WordPress. See #16413, #18801, #39441, and the explorations made on ​https://github.com/wpaccessibility/settings-api-enhanced.

That said, there are a few cases where the semantics of these forms within tables can be improved. I'm working on a patch to address these cases.

[afercia]

47390.diff​ does the following:

• adds role="presentation" to the edit comment layout table
• removes a few pointless <fieldset> elements
• adds a few missing <label> elements, especially on some network admin pages
• adds the CSS class class="td-full" to table rows spanning to multiple cells
• adds explicit scope="row" attribute to the table headers in options-permalink.php: this table is better communicated as data table
• consistently use label association in the "Privacy Settings" page, see attached screenshots
• in the installation "Set up your database connection" page: associates descriptions to their inout fields using aria-describedby
• improves the link to gravatar.com in the user-edit.php page

Re: the "Privacy Settings" page, the select label should be the bold text in the left column. I'd like to propose to remove Select an existing page: and just use the left column text as the <label>, also for consistency with all the similar settings tables in the admin. See attached screenshots.

[afercia]

select privacy page before

[afercia]

select privacy page after

[afercia]

change privacy page before

[afercia]

change privacy page after

[afercia]

In 45416:

Accessibility: Improve settings tables and forms after [45403].

• adds role="presentation" to the edit comment table
• removes a few pointless <fieldset> elements
• adds a few missing <label> elements
• adds the CSS class class="td-full" to table rows spanning to multiple cells
• adds explicit scope="row" attribute to the table headers in options-permalink.php: this table is better communicated as data table
• uses consistent label association in the "Privacy Settings" page
• in the installation page "Set up your database connection": associates descriptions to their inout fields using aria-describedby
• improves the link to gravatar.com in the user-edit.php page

See #46899.
Fixes #47390.

[JeffPaul]

Reopening this so it can be back-ported to the 5.2 branch, also needs testing to validate if this ticket is good to land in 5.2.3.

[SergeyBiryukov]

In 45835:

Accessibility: Improve settings tables and forms after [45403].

• adds role="presentation" to the edit comment table
• removes a few pointless <fieldset> elements
• adds a few missing <label> elements
• adds the CSS class class="td-full" to table rows spanning to multiple cells
• adds explicit scope="row" attribute to the table headers in options-permalink.php: this table is better communicated as data table
• uses consistent label association in the "Privacy Settings" page
• in the installation page "Set up your database connection": associates descriptions to their inout fields using aria-describedby
• improves the link to gravatar.com in the user-edit.php page

Props afercia.
Merges [45416] to the 5.2 branch.
See #46899.
Fixes #47390.