Implement friends-only indicator (#1104)

[huntertur]

Implements #1104.

Some notes:

• I added the friends-only indicator everywhere the existing mature/explicit indicator exists. The exception is in the critique section of the homepage, since I believe the underlying SQL query there explicitly filters out all friends-only submissions, even if the current user can see them?
• Since this is less important than the mature/explicit indicator, I made the new one about 2/3 the size of the existing one. This also solves a space constraint problem when displaying journals.
• I noticed that a user's journal page (/journals/username) doesn't currently show mature/explicit indicators, so I left that page alone. Maybe I could open an issue for that one and take a look into adding indicator support in general as a separate item.
• I chose pink instead of eyedropper-picking the existing color for Friends Only because I feel it is too close to the color used by the explicit indicator and could be therefore be confusing.

Examples of how this looks

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 61.55%. Comparing base (88c8630) to head (6ee4449).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1433   +/-   ##
=======================================
  Coverage   61.55%   61.55%           
=======================================
  Files          96       96           
  Lines        9219     9219           
  Branches     1659     1659           
=======================================
  Hits         5675     5675           
  Misses       3085     3085           
  Partials      459      459           

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[philpem]

I noticed that the https://www.weasyl.com/journals/(username) doesn't show the rating or friends-only status of journals. If @charmander thinks it's needed, it'd be nice to get it in on this PR.