Git tag feature

[chrisjohn2306]

The changes for git tag feature has been added in this PR.
#638 is already existing. But due to some conflict issues the pr has been corrupted. The review comments from that pr has been incorporated here.

The feature discussion is in #588

[chrisjohn2306]

@fcollonval and @kgryte . Can you review the changes again and help me to complete this feature

[fcollonval]

@chrisjohn2306 Thank you for this feature.

I simplified the frontend code to:

• Use SVG icon
• Take advantage of the new status bar and alert components instead of using two dialogs.
• Use more homogeneous signature of model method (i.e. not passing the repo path as argument)

[fcollonval]

Ok after some fight with the latest notebook version and the unit test. This is good to go.

[fcollonval]

@meeseeksdev backport to 0.11.x

[lumberbot-app]

Owee, I'm MrMeeseeks, Look at me.

There seem to be a conflict, please backport manually. Here are approximate instructions:

1. Checkout backport branch and update it.

$ git checkout 0.11.x
$ git pull

2. Cherry pick the first parent branch of the this PR on top of the older branch:

$ git cherry-pick -m1 c1ffae2f894fb455cca5c636f2f64c9037784a8c

3. You will likely have some merge/cherry-pick conflict here, fix them and commit:

$ git commit -am 'Backport PR #713: Git tag feature'

4. Push to a named branch :

git push YOURFORK 0.11.x:auto-backport-of-pr-713-on-0.11.x

5. Create a PR against branch 0.11.x, I would have named this PR:

"Backport PR #713 on branch 0.11.x"

And apply the correct labels and milestones.

Congratulation you did some good work ! Hopefully your backport PR will be tested by the continuous integration and merged soon!

If these instruction are inaccurate, feel free to suggest an improvement.