web: on forum page, tell user they have to log in to post

[davidpanderson]

Currently, if we direct someone to the "Questions and problems" forum,
and they're not logged in, it's a dead end;
they aren't told that they need to log in
(and/or create an account) in order to post a question. Add this.

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 11.95%. Comparing base (1e99ff5) to head (e6ef943).
Report is 17 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #6040      +/-   ##
============================================
- Coverage     11.95%   11.95%   -0.01%     
  Complexity     1068     1068              
============================================
  Files           280      280              
  Lines         36974    36982       +8     
  Branches       8533     8533              
============================================
  Hits           4420     4420              
- Misses        32154    32162       +8     
  Partials        400      400              

see 4 files with indirect coverage changes