Adds support for versioned XSLT files

[mauromsl]

• Adds a new model for XSL files to core
• Installs the default.xsl file as the first XSLFile object.
• Sets the default.xsl as the default for all journals via migration
• Sets the default.xsl as the preferred XSL for all Galleys via migration
• Adds interface for uploading new XSL, choosing the journal default and changing galley XSL
• Adds a json dump of the default XSLFile object to be loaded at install time

Considerations:

• I tried to make XSLFile a wrapper around core.File similar to how SupplementaryFile was implemented. The problem with this approach is that the File model is too coupled with the Article model, making it messy to save files under that model when they are not related to an article.
• At first I linked the XSLFile to articles, but I think it makes more sense to link them to the Galleys themselves, when these are XML (galley.label == 'XML').
• Changes to the default.xsl file should not be allowed and should instead generate a new file + a data migration whenever changes are required)