Make comments an annotation layer

[mbollmann]

Originally reported by: Marcel Bollmann (Bitbucket: mbollmann, GitHub: mbollmann)

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Comments (of the type that is displayed in the editor) are currently treated differently from other annotations, although logically they behave in the same way: they are attached to mods, they appear in the export just like any other annotation, etc.

Consequently, they should be treated as such internally, i.e.:

• There should be a tagset of type "comment", which can be linked to texts
• Existing comments (= entries in the "comment" table of type "C") need to be converted to the new representation
• The "subtok_id" column of the "comment" table would no longer be needed

The change is facilitated by the fact that the JavaScript side already treats "comment" like any other tagset; the main change would be on the server-side code.

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• Bitbucket: https://bitbucket.org/mbollmann/cora/issue/38

[mbollmann]

Original comment by Marcel Bollmann (Bitbucket: mbollmann, GitHub: mbollmann):

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Convert internal comments to annotations; resolve #38

This change converts internal comments, previously stored in table comment with comment_type="C", to a new annotation layer with class="comment".

The change gets rid of various comment-specific code, since these comments are now treated like any other annotation. It is now possible to have texts with a comment column, if desired.

In XML, comments are now exported as instead of

..., but for importing, the old representation is still accepted.