Add MirrorInterval to the API

[paulbarton90]

This should address #11605. Allows you to create a migration setting the value, and edit it after the fact.

[codecov-io]

Codecov Report

Merging #14163 (658da8d) into master (cf9d471) will decrease coverage by 0.36%.
The diff coverage is 6.49%.

@@            Coverage Diff             @@
##           master   #14163      +/-   ##
==========================================
- Coverage   42.36%   42.00%   -0.37%     
==========================================
  Files         728      733       +5     
  Lines       78015    78756     +741     
==========================================
+ Hits        33049    33079      +30     
- Misses      39540    40239     +699     
- Partials     5426     5438      +12     

Impacted Files	Coverage Δ
cmd/dump_repo.go	0.00% <0.00%> (ø)
cmd/restore_repo.go	0.00% <0.00%> (ø)
models/action.go	50.93% <0.00%> (-1.98%)	⬇️
models/admin.go	60.31% <0.00%> (-8.78%)	⬇️
models/repo.go	52.15% <ø> (+0.25%)	⬆️
models/repo_list.go	78.76% <0.00%> (-0.71%)	⬇️
models/task.go	34.95% <ø> (+0.04%)	⬆️
modules/auth/repo_form.go	39.83% <ø> (ø)
modules/migrations/base/downloader.go	17.51% <ø> (ø)
modules/migrations/base/pullrequest.go	0.00% <ø> (ø)
... and 39 more

---

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[zeripath]

@6543 Please could you re-review.

[6543]

jup

[techknowlogick]

🚀