Ensure that sessions are passed into queries that could use the database to prevent deadlocks

[zeripath]

I've looked through the gitea code following the use x.NewSession() checking whether every request is made within the session in an attempt to prevent a deadlock.

This PR accounts for most of the potential deadlocks I've found in those requests.

Issues

• Although https://github.com/go-gitea/gitea/blob/master/models/issue.go#L1349 opens a session its call to IssuesList.loadAttributes cannot be called within the session because of an unusual bug Session not completely cleaned  go-xorm/xorm#1185 . This doesn't need to happen within the session as this session is not a transaction
• models/repo_mirror.go:SyncMirrors() doesn't pass its session down to its children calls however, it doesn't create a transaction (despite updating the db) - this is likely reasonable as the transaction would lock the db whilst these somewhat long actions were ongoing. This could be refactored to have an ongoing mirror action flag in the db (changed and checked with a transaction of course) that would be cleaned up at the end of the method if data consistency was at risk from this. (I haven't looked deep into this and this is likely a non-issue.)

[codecov-io]

Codecov Report

Merging #5718 into master will increase coverage by <.01%.
The diff coverage is 53.12%.

@@            Coverage Diff             @@
##           master    #5718      +/-   ##
==========================================
+ Coverage   37.75%   37.76%   +<.01%     
==========================================
  Files         325      325              
  Lines       47640    47650      +10     
==========================================
+ Hits        17988    17994       +6     
- Misses      27058    27062       +4     
  Partials     2594     2594

Impacted Files	Coverage Δ
models/issue_comment.go	46.66% <0%> (ø)	⬆️
models/org.go	69.3% <100%> (+0.12%)	⬆️
models/pull.go	50.75% <37.5%> (ø)	⬆️
models/issue.go	47.64% <40%> (-0.09%)	⬇️
models/repo.go	43.88% <55.55%> (+0.01%)	⬆️
models/star.go	56.09% <80%> (+1.09%)	⬆️

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