Support deleting and/or rewriting tags

As far as I can tell, there's no way to move a tag from one commit to another within lazygit. I think this would be a nice feature which could be accomplished in a few ways.

1. Opening the `T` menu on a commit that already has a tag includes an option to delete the tag.
2. tags are always created with the -f option (which will move the tag if it existed before)
3. when trying to use an existing tag, a dialogue box appears asking the user if they'd like to move the tag from the other commit.

I don't think 2 is a good idea, but including for completeness. 1 and 3 might both be nice actually.