[RTM] Associate comments with members

[m-vo]

This PR adds a field member to tl_comments which receives the logged in member's id when a comment is stored (or null if the users is a guest).

see #5

I did not add anything to the templates for BC reasons (and: old comments would have the member id 0 set and need some manual migration) nor did I add it explicitly to the template's vars. So this is kind of a hidden feature but would at least allow something like the following in a custom version of the com_default template:

$member = \MemberModel::findById($this->arrData['member']);
if($member) { echo $member->...; }

This allows for several things like displaying user avatars or additional info in comments, listing comments in a user dashboard and so on.

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Remarks:

• I don't like the name member. This field links the actual author of the comment and should imo be called something like author or authorProfile but this term is already used for the author of the reply.
• Note that the data stored within a comment (email, name, ..) could differ from the one in the member profile.

[leofeyer]

As discussed in Mumble on October 18th, member is ok as column name.

[m-vo]

As discussed on mumble the member now gets displayed in the backend if available:

(We could also add the information in the tl_comment label callback, but it's is already a lot in there and a duplicate author/member could be confusing.)

@leofeyer Anything else to add/modify?

[leofeyer]

Is there really a use-case in which a comment needs to be re-assigned from one member to another? I don't think the field needs to be in the back end form.

[m-vo]

Probably not, but there could be the use-case to assign comments that have been made without the member field present (<4.7) or that were issued without login to a member later on.

[leofeyer]

Merged in contao/contao@e8002e0767d2d7ecc4f71ee2e323d5705dc0b8e4.