Send push event when tag is created or deleted

[CirnoT]

#13381 introduced behavioral change in which push event with 0 commits was no longer sent when tag is created or removed. This is necessary:

• To ensure compatibility with all CI solutions
• To create advanced triggers for Drone (by allowing to trigger on refs/tags/**)
• To maintain compatibility with GitHub webhooks

Similar fix was already made for mirrors in #13862

[CirnoT]

To create advanced triggers for Drone (by allowing to trigger on refs/tags/**)

For example:

trigger:
    ref:
        - refs/heads/master
        - refs/pull/**
        - refs/tags/**

will trigger on push to master, PR or tag creation (on event push, ref name is refs/tags/v3.1.0), however

trigger:
    ref:
        - refs/heads/master
        - refs/pull/**
        - refs/tags/**
    event:
        - push
        - tag

will only trigger on push to master and PR creation, because on event tag, ref value is v3.1.0, not refs/tags/v3.1.0 (note that all conditions must be true for trigger to fire)

[CirnoT]

Related #11397

[codecov-io]

Codecov Report

Merging #13999 (b121fdd) into master (b8c58ed) will decrease coverage by 0.03%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master   #13999      +/-   ##
==========================================
- Coverage   42.23%   42.19%   -0.04%     
==========================================
  Files         710      710              
  Lines       77261    77275      +14     
==========================================
- Hits        32629    32609      -20     
- Misses      39266    39288      +22     
- Partials     5366     5378      +12     

Impacted Files	Coverage Δ
services/repository/push.go	38.46% <0.00%> (-4.65%)	⬇️
modules/notification/ui/ui.go	93.05% <0.00%> (-2.78%)	⬇️
modules/process/manager.go	72.50% <0.00%> (-2.50%)	⬇️
services/pull/check.go	49.63% <0.00%> (-2.19%)	⬇️
models/notification.go	66.25% <0.00%> (-0.99%)	⬇️
modules/log/event.go	58.96% <0.00%> (-0.95%)	⬇️
models/error.go	38.49% <0.00%> (-0.82%)	⬇️
services/pull/pull.go	40.68% <0.00%> (-0.50%)	⬇️
models/issue_comment.go	52.26% <0.00%> (-0.46%)	⬇️
modules/notification/mail/mail.go	39.08% <0.00%> (ø)
... and 2 more

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