WIP: Move migration git clone to its own goroutine

[zeripath]

The current code places the git clone in the POST goroutine, blocking that goroutine until it is finished. This PR asynchronises this, placing the clone within its own goroutine. Fix #3770. Fix #1249.

[zeripath]

• migration goroutine needs panic protection with a recover.
• need to think about how migration failures are notified. I currently attach these in an action to the repo, but these should be deleted when the repo is... I think there must be a better way of informing errors etc. Turns out this is not possible.. Therefore if the migration fails I will keep the repository around
• need to fix tests
• rebase on head fix conflicts

[codecov-io]

Codecov Report

Merging #5949 into master will increase coverage by 0.04%.
The diff coverage is 48.33%.

@@            Coverage Diff            @@
##           master   #5949      +/-   ##
=========================================
+ Coverage   38.86%   38.9%   +0.04%     
=========================================
  Files         345     346       +1     
  Lines       49510   49670     +160     
=========================================
+ Hits        19241   19324      +83     
- Misses      27486   27553      +67     
- Partials     2783    2793      +10

Impacted Files	Coverage Δ
models/action.go	60.98% <ø> (ø)	⬆️
modules/templates/helper.go	48.95% <0%> (ø)	⬆️
models/repo.go	46.83% <28.57%> (-0.45%)	⬇️
models/repo_migrate.go	47.98% <47.98%> (ø)
routers/repo/repo.go	21.4% <53.84%> (+0.58%)	⬆️
routers/api/v1/repo/repo.go	56.79% <83.33%> (+0.15%)	⬆️
modules/util/sanitize.go	63.15% <0%> (+63.15%)	⬆️

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[zeripath]

Ok, this is now working. In terms of things that could be done:

• At present I just create an empty repository and archive it as a placeholder for the migration. It may be possible to set something in the repository to show it is in migration?
• I haven't checked what happens should you try to delete this repository whilst it is in migration. That may be horrible... It was horrible - this is now fixed.
• It may actually make sense to try to recover as much as possible from a failed migration rather than just delete failed clones.

[zeripath]

Closing this PR in preference to lunny's