[#162] Fix update form in RSR

Add the language field to the project update form in rsr,
templates/rsr/project/update_form.html, that was recently added in the
ProjectUpdate model.

Add help text to the language field in both forms

Add dev domain in partner sites middleware
[#164] Fix middleware for CNAME access of partner sites

Fix runtime error happening when a partner site is requested using a
CNAME domain
Update release notes for hotfix/2.1.3.1
Merge branch 'hotfix/2.1.3.1'