Added Markdown formatting in all the fields of the "About" page

[Poslovitch]

Description

This PR introduces Markdown formatting in additional fields of the instance's "About" page.
It replaces plain-text textareas in the admin configuration by my-markdown-textarea as well as adds the <my-help helpType="markdownText"></my-help> to all of them.
It also registers the missing fields into instance.service.ts so that their content gets rendered as Markdown.

Related issues

Resolves #3368.

Has this been tested?

• 👍 yes, I added tests to the test suite
• 👍 yes, light tests as follows are enough
• 💭 no, because this PR is a draft and still needs work
• 🙅 no, because they aren't needed
• 🙋 no, because I need help

Since my computer is a bit of a potato, I can't efficiently run Docker on it. Therefore, I developed all of that on Gitpod.

The "new" code reuses the same "code" that previously existed. It was just a matter of tweaking some HTML tags and adding a few fields in instance.service.ts. I tested the code by running a dev PeerTube instance and modifying the "config" of the About page a few times (this included: not-formatted text, Markdown-formatted text, blank text fields...).

Screenshots

Proof that the Markdown formatting now works in fields it previously didn't:

[Poslovitch]

@kimsible textareaHeight is not defined for others my-markdown-textarea (those that were already there prior to this PR). Do you think I should also add it to them?

[kimsible]

@kimsible textareaHeight is not defined for others my-markdown-textarea (those that were already there prior to this PR). Do you think I should also add it to them?

@Poslovitch My bad, sorry, the default textareaHeight for markdown is 150px:

PeerTube/client/src/app/shared/shared-forms/markdown-textarea.component.ts

Line 26 in 4a7f902

@Input() textareaHeight = '150px'

Though this PR adds markdown to all about textarea I think you can actually delete them, 75px looks very small with the preview.

[kimsible]

Thanks for the PR @Poslovitch ! LGTM 👍 !

[Chocobozzz]

Thanks @Poslovitch

[Poslovitch]

Thank you too 🙂