Fix #2744

* Fix header button changing header size in settings pages

* Make form buttons look more like a part of the form in settings pages

- Put buttons closer, using same distance as between inputs
- Make buton font size a bit smaller to blend a bit more
- Add the class button to button tags for consisent styling

* Display appropriate error when performing unpermitted operation on custom emoji

Fixes #13897

* Remove links to custom emoji actions not performable by moderators

* Fix unused role routes being generated

* Remove unused JavaScript code

* Refactor filters code to be DRYer

* Fix `.count == 0` comparisons to `.empty?` in views

* Fix filters in views

This commit:

- Refactors centered text blocks currently used for placeholders
  for empty tables and puts styles for it in separate class -
  .centered-text, simply aliasing text-align: center. Which is
  furtherly used in this commit.

- Improves applications settings page to use heading actions, moving
  "New application" button there, and displaying placeholder "You
  have no applications" in place of empty table.

- Improves custom emoji settings page to use heading action
  for "Upload" button, making it more easily accessible without
  need to scroll through all of the emojis.

- Improves email domain blocks settings page, moving "Add new" to the
  heading actions and using placeholder "No e-mail domains currently
  blacklisted" instead of showing empty table.

Fix #12132

Fixes #9535

* Verify link ownership with rel="me"

* Add explanation about verification to UI

* Perform link verifications

* Add click-to-copy widget for verification HTML

* Redesign edit profile page

* Redesign forms

* Improve responsive design of settings pages

* Restore landing page sign-up form

* Fix typo

* Support <link> tags, add spec

* Fix links not being verified on first discovery and passive updates

* Show the local couterpart of emoji when it exists in admin/custom_emojis

* Fix indentation

* Fix error

* Add class table-action-link to Overwrite link

* Make it enable to overwrite emojis

* Make Code Climate happy

* Fix #5351 - Redirection with page params after copy emoji

* Add page params to copy emoji link

* Admin interface for listing, adding and removing custom emojis

* Only display local ones in the list