Make Slack fallback messages more compressed

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Description

Currently the fallback message (when using the IRC gateway to connect to Slack) for a push can look like this:

                   https://git.url.com/group/project/compare/a3eb49cb11b24a651b096f8235bfd848b7ab5902...111d9e7d5e79613c4460d6524a72b0189a18fde3 )
23:10:06 < GitLab> 111d9e7d https://git.url.com/group/project/commit/111d9e7d5e79613c4460d6524a72b0189a18fde3 : Add slack thingy
23:10:06 < GitLab>  - John Doe```

However, all of our developers using Slack through IRC find the presentation of this information to be too much because of all the links takes away from the messages.

### Proposal

Consider to make it contain less text for the IRC fallbacks and reorganize the order slightly. You may not only want to review pushes of commits, but also the other actions that can trigger a push. For example we disabled the wiki-update messages, because the IRC fallback had it contain all of the text.

For example, instead of the above, it could be like: 

```23:10:06 < GitLab> John Doe pushed to branch master  of group/project https://git.url.com/group/project
23:10:06 < GitLab> 111d9e7d: Add slack thingy (John Doe)
23:10:06 < GitLab>  Compare changes 
                   https://git.url.com/group/project/compare/a3eb49cb11b24a651b096f8235bfd848b7ab5902...111d9e7d5e79613c4460d6524a72b0189a18fde3 ```

I think this fallback would look more organized when viewed through the IRC gateway. I also think you could drop the Compare changes link with little complaints (you have a link to go to the project if you want to above, where it should be easy enough to find the comparisons etc.), but as long as it's moved to the last line (or at least it's own line) I think it would at least flow a bit better. 

The author information from the last line has here been moved together with the commit message. (But I'm not sure if there is cases it would be different from the author information provided on the first line, so it could maybe be cut?)


### Links / references
Edited by 🤖 GitLab Bot 🤖