Drop Markdown support in bio field

What does this MR do?

This MR drops the support for Markdown in bio field on user profile page and in user pop-over. Read the discussion below to understand why; in short - supporting Markdown makes bio vulnerable and poses security concerns.

Note: Users API was exposing both bio and bio_html.

Screenshots or Screencasts (strongly suggested)

How to setup and validate locally (strongly suggested)

1. Visit user profile page http://127.0.0.1:3000/[username]
2. In case you have no text in bio field yet, go to Edit profile page, add whatever you please, save the changes
3. Visit again user profile page

Conformity

• I have included changelog trailers, or none are needed. (Does this MR need a changelog?)
• I have added/updated documentation, or it's not needed. (Is documentation required?)
• I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• I have added information for database reviewers in the MR description, or it's not needed. (Does this MR have database related changes?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.
• This change is backwards compatible across updates, or this does not apply.

Availability and Testing

• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)
• I have tested this MR in all supported browsers, or it's not needed.
• I have informed the Infrastructure department of a default or new setting change per definition of done, or it's not needed.