[issue - 28, 67, 68, 72,73] improvements in bitbucket subscriptions

[hanzei]

Summary

Resubmission of https://github.com/mattermost/mattermost-plugin-bitbucket/pull/75

Ticket Link

Fixes #67 Fixes #68 Fixes #72 Fixes #73

[hanzei]

@mickmister This is a re submission of https://github.com/mattermost/mattermost-plugin-bitbucket/pull/75 with master merged.

[codecov-commenter]

Codecov Report

Base: 16.12% // Head: 16.19% // Increases project coverage by +0.07% :tada:

Coverage data is based on head (2b19bd4) compared to base (650ec0f). Patch coverage: 0.00% of modified lines in pull request are covered.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #97 +/- ## ========================================== + Coverage 16.12% 16.19% +0.07% ========================================== Files 13 13 Lines 1861 1852 -9 ========================================== Hits 300 300 + Misses 1542 1533 -9 Partials 19 19 ``` | [Impacted Files](https://codecov.io/gh/mattermost/mattermost-plugin-bitbucket/pull/97?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=mattermost) | Coverage Δ | | |---|---|---| | [server/command.go](https://codecov.io/gh/mattermost/mattermost-plugin-bitbucket/pull/97/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=mattermost#diff-c2VydmVyL2NvbW1hbmQuZ28=) | `6.97% <0.00%> (+0.31%)` | :arrow_up: | | [server/subscriptions.go](https://codecov.io/gh/mattermost/mattermost-plugin-bitbucket/pull/97/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=mattermost#diff-c2VydmVyL3N1YnNjcmlwdGlvbnMuZ28=) | `14.17% <0.00%> (-0.35%)` | :arrow_down: | Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=mattermost). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=mattermost)

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