Fix digital source URL

[cmil]

For each play we currently provide two URLs for the digital source, one is an HTML page and the other one is the actual TEI from Théâtre Classique. See for instance:

fredracor/tei/abeille-argelie.xml

Lines 28 to 31 in a9fa55c

	<bibl type="digitalSource">
	<name>Théâtre Classique</name>
	<idno type="URL">http://theatre-classique.fr/pages/programmes/edition.php?t=../documents/ABEILLE_ARGELIE.xml</idno>
	<idno type="URL">http://theatre-classique.fr/pages/documents/ABEILLE_ARGELIE.xml</idno>

While the issue #22 caused by this has been fixed on the API level, we should still stick to only one source URL. I would suggest to loose the reference to the HTML page and just keep the TEI URL, since these are actually the documents we use for the import to FreDraCor. What do you think @lehkost?

See original post by @cmil in #22 (comment)

[lehkost]

The HTML reference has the advantage to show a fully rendered page, but I admit, there is no real use case for this, so yes, let's stick to just one source url, the one being the XML-TEI version.