Add a field to specify reviewers when submitting a new feed item

[Xpirix]

• This PR is addressing the issue Clarify who are the reviewers for a feed submission #57

Changes summary

• Implement chosen script and styles
• Add a new field Reviewers on the new feed item form
• The author can choose one or multiple reviewers
• If at least one reviewer is specified, set the specified reviewers as recipients and cc the other reviewers. Otherwise, set all reviewers as recipients of the email.

Requirements

• Only superusers and users with the group permission qgisfeedentry_approver can review a feed item
• We should ensure that the variables for email sending are configured correctly on the server side

Please find below a GIF demonstrating the new field:

Please find below a screenshot of the email for reviewers:

[haubourg]

that's perfect to me 👍

[dimasciput]

@Xpirix looks good, but please could you add test for this. Thanks.

[Xpirix]

My pleasure. I've added the test for this.

[dimasciput]

@Xpirix LGTM, please fix the conflicts

[Xpirix]

@dimasciput , The conflicts are fixed.

[Xpirix]

This PR has been deployed. The email sending will not work yet. We will set up the variables to make it work.