Re-enable listing of forks when logged out

[CL-Jeremy]

This PR addresses point 1 of #14926 further by further aligning the look-and-feel of the fork button to that of the other buttons.

When hovering above the fork button, a guest user will still be prompted to log in, but no action is performed on mouse click, exactly like the other two buttons. The user will still be able to list forks when logged out.

[CL-Jeremy]

CanSignedUserFork gives false when User is nil, so the or clause is redundant. The naming of the property confused me a little.

[codecov-io]

Codecov Report

Merging #14992 (26d5028) into master (487f2ee) will increase coverage by 0.04%.
The diff coverage is 40.67%.

@@            Coverage Diff             @@
##           master   #14992      +/-   ##
==========================================
+ Coverage   42.21%   42.25%   +0.04%     
==========================================
  Files         767      774       +7     
  Lines       81624    82994    +1370     
==========================================
+ Hits        34458    35073     +615     
- Misses      41531    42224     +693     
- Partials     5635     5697      +62     

Impacted Files	Coverage Δ
cmd/dump.go	0.93% <0.00%> (-0.01%)	⬇️
cmd/serv.go	2.61% <0.00%> (-0.02%)	⬇️
cmd/web.go	0.00% <0.00%> (ø)
cmd/web_graceful.go	0.00% <0.00%> (ø)
cmd/web_letsencrypt.go	0.00% <0.00%> (ø)
models/action.go	48.52% <0.00%> (-2.42%)	⬇️
models/migrations/migrations.go	2.59% <ø> (ø)
models/migrations/v130.go	0.00% <0.00%> (ø)
models/migrations/v166.go	0.00% <0.00%> (ø)
models/migrations/v172.go	0.00% <0.00%> (ø)
... and 190 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update f5aaa74...9a623e5. Read the comment docs.

[CL-Jeremy]

The second commit is admittedly a bit hacky, for enabling cursor events from outside (div.labeled.button) while disabling from inside (button/a.basic.button, the former is done by Semantic UI).

This revision would ensure that:

• pop-ups always come from outer element
• disabled elements show an arrow as cursor
• disabled elements do not react to mouse events (previously, a disabled .labeled.button > .button reacts to double-click and the inner text gets selected - very fine detail, but annoyed me a bit)
• a non-reactive button is always disabled (wasn't the case when logged in and unforkable, i. e. own repo)

I personally find the low opacity a bit intimidating, but probably that's just me.