Make the visual editor KSES filter/user capabilities aware

[azaozz]

Currently the visual editor assumes "unlimited html" for all users. This can result in some surprises when a user doesn't have that capability.

We can make a function that translates the KSES allowed tags and attributes into TinyMCE format and sets the editor accordingly.

[chriscct7]

@azaozz Do you know if there a function for this now? I'm almost certain there is.

[azaozz]

It is possible to do even better/easier in TinyMCE 4.0. We can set the allowed_elements without worrying that some internals would break.

However the usefulness of this is quite limited. Kses was fixed/expanded several times since this ticket. Generally by default the users cannot add "forbidden" tags and attributes, so this would apply only for plugins that extend the editor and only in certain cases. Also, we cannot stop the users adding arbitrary content in the Text editor.

Thinking this is something we can revisit in the future if a need arises.