Improve data structure

[pfefferniels]

This rather huge (sorry for that) PR:

• changes the way the data folder as well as the URL scheme is structured into [number]/[author]/[edition], e.g. 1/mattheson/secondEdition for the original score and text of Johann Mattheson in the second edition, or e.g. 3/hill/english for a comment by Robert Hill given in English etc.
• allows referencing particular measures or paragraphs by appending a #-fragment to the URL, e.g. 14/mattheson/secondEdition#p1-1 to jump to the first paragraph in the comments or 14/mattheson/secondEdition#note-0000001247342959 to jump to a particular note in the 14th Probstück
• allows displaying only score or only text, depending on the files given in the data folder
• Adds a new facsimile-router. E.g., facsimile/1/firstEdition will display the full-page facsimile of the first Prob-Stück in the first edition using a Mirador instance (based on information retrieved from an IIIF annotation list)
• renames "annotation" TEI files to "comments" in order to avoid confusion with the IIIF annotation lists
• uses bootstrap in order to have a more responsive and easy-to-manage layout
• Adds additional material, e.g. a realization of Prob-Stück 10, some lesson material for Prob-Stück 3 etc.
• encodes the front page as a TEI file
• removes a lot of unnecessary, redundant or useless code

An instance of this branch is currently viewable here.

@rettinghaus I want to merge this as soon as possible into master, in order to avoid possible merge conflicts with your work (mostly since this is also affecting the score.mei files). I'll be happy about any comments …

[rettinghaus]

I moved my work on PS5 to a separate branch, so I can deal with that later. That leaves us only two conflicts (PS1 and PS2). So you should merge my PR first, then yours, and try to deal with the conflicting files. If something gets lost on the way, I think we both could reproduce our work on those files.

[rettinghaus]

Strange … My tests had different results.

[pfefferniels]

github is wrong here, I think. As predicted there are only merge conflicts in PS 1 and 2 when I merge with git directly