Customize Themes: update dashboard welcome link to use themes in the customizer

[celloexpressions]

Now that the customizer version of themes provides a more seamless flow for exploring and customizing themes, let's update the "or, change your theme completely" text in the dashboard welcome panel to link there. This way, once a new user clicks this link, they can stay within the customization context throughout the experience of selecting/previewing/customizing themes, and set up their site's design without returning to wp-admin.

[celloexpressions]

Deep-link to themes in the customizer, instead of wp-admin installed themes.

[melchoyce]

Let's do it!

[westonruter]

In 41699:

Customize: Update dashboard welcome link to point to themes panel in the customizer instead of themes admin screen.

Props celloexpressions.
Fixes #42050.

[westonruter]

In 42169:

Customize: Prevent PHP notice on theme-switch dashboard welcome link (and link to themes admin screen) when user cannot customize.

Props skostadinov, ocean90.
Amends [41699].
See #42050.
Fixes #42525.

[westonruter]

In 42170:

Customize: Prevent PHP notice on theme-switch dashboard welcome link (and link to themes admin screen) when user cannot customize.

Props skostadinov, ocean90.
Amends [41699].
See #42050.
Fixes #42525 for 4.9.