ux: add hashtag suggestions in compose

Addresses #2169 (closed) #1830

Notes:

• This MR also addresses a pre-existing issue where the actual insertion of the expanded suggestion is broken (see #1830 as an example). Relevant changes are in 8577ce17, which also includes a change in the behavior of dismissing the suggestions menu. Before, we'd try to do some text insertion and selection change in addition to dismissing the menu, but thought it made more sense to only dismiss the menu and not try to change anything about the post text

TODO:

• make sure there aren't any regressions for mentions suggestion functionality due to refactor
• more testing (both desktop + mobile)
• potentially fix #2118 (closed) see !378 (merged)
• potentially fix pre-existing issue when cancelling suggestion that causes erroneous text to be inserted into doc See 8577ce17
• potentially fix pre-existing issue where typing newlines after a potentially autocompletable entity doesn't dismiss the suggestions menu.
• add e2e test

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Preview:

• Desktop:

  

• Mobile: