Add cron jobs

[6543]

TODOs

Backend

• Move Server Pipeline Build code out of api into own package #949 got merged
• use https://github.com/robfig/cron to parse next schedule
• add api
• scheduler work
• set a default event filter (push, tag, pull, deploy) Add default event filter #1140
• add "cron" name filter

Frontend

• add ui
• mention time is in UTC
• Set clock-icon as avatar if cron task
• hide refs/heads/ part in build-list-view as done with others

CLI

• extend client sdk
• add cli subcomand
• ls works
• add works
• update works
• info works
• rm works

Docs

• add docs
• mention default event filter (push, tag, pull, deploy)

Tests

• unit tests
• integration tests

Related refs

• https://docs.drone.io/pipeline/docker/syntax/trigger/#by-cron
• handle timezones (https://blog.fuzzymistborn.com/drone-love-hate/)

Follow UP

• Button to trigger a run now

close #8

[codecov-commenter]

Codecov Report

Merging #934 (358384f) into master (08a9915) will decrease coverage by 1.02%.
The diff coverage is 16.41%.

@@            Coverage Diff             @@
##           master     #934      +/-   ##
==========================================
- Coverage   50.40%   49.38%   -1.03%     
==========================================
  Files          83       86       +3     
  Lines        6341     6535     +194     
==========================================
+ Hits         3196     3227      +31     
- Misses       2959     3120     +161     
- Partials      186      188       +2     

Impacted Files	Coverage Δ
server/model/build.go	0.00% <ø> (ø)
server/model/cron.go	0.00% <0.00%> (ø)
server/remote/bitbucket/bitbucket.go	85.71% <0.00%> (-1.17%)	⬇️
server/remote/bitbucketserver/bitbucketserver.go	0.00% <0.00%> (ø)
server/remote/coding/coding.go	62.96% <0.00%> (-0.89%)	⬇️
server/remote/gitea/gitea.go	34.27% <0.00%> (-1.42%)	⬇️
server/remote/github/github.go	15.30% <0.00%> (-0.42%)	⬇️
server/remote/gitlab/gitlab.go	22.43% <0.00%> (-0.88%)	⬇️
server/remote/gogs/gogs.go	50.23% <0.00%> (-2.50%)	⬇️
server/store/datastore/migration/migration.go	37.50% <ø> (ø)
... and 4 more

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[qwerty287]

Didn't find typos otherwise :)