Resolve "Names of subjects should not have to be unique"

Closes #158

See merge request !221

[Fixtures] Fix number and period of validity ranges

See merge request !220

Remove admin user from test data

See merge request !219

Add trademark hint to readme

See merge request !218

Resolve "Allow editing of subject field on Group model in frontend"

Closes #156

See merge request !215

Resolve "Announcements are not shown on substitutions printout"

Closes #157

See merge request !216

Update dev dependencies and reformat

See merge request !217

Merge branch '155-automaticplan-doesn-t-look-at-versions-between-the-last-revision-and-the-current-revision' into 'master'

Resolve "AutomaticPlan doesn't look at versions between the last revision and the current revision"

Closes #155

See merge request !214

Add makemessages env to tox.ini

See merge request !213

Merge branch '154-nameerror-at-app-resint-live_documents-pdf-name-timedelta-is-not-defined' into 'master'

Resolve "NameError at /app/resint/live_documents/*.pdf: name 'timedelta' is not defined"

Closes #154

See merge request !212

Bump version to 2.1.dev0

See merge request !211

Resolve "Automatically create a PDF file of the substitution plan"

Closes #116

See merge request !191

Resolve "Events replace lessons"

Closes #127

See merge request !207

Update changelog from 2.0

See merge request !210

(cherry picked from commit 3dd7f9f3)

Fix changelog order

See merge request !208