If welcome is enabled, use it to recover from a forgotten password (Closes hhucn/adhocracy.hhu_theme#243)
Improve password reset message when welcome_back is enabled
Remove useless files.

To my surprise, adhocracy works perfectly fine without these files. The only reference I can find is to plain.css
Makek massmessage footer optional.

Currently, every message has the unsubscription footer. That's fine (even required) in open communities, but out of place if there is some kind of central user management (say, in a company), and if non-technical users may not even be aware we're using adhocracy as a newsletter service.
This commit therefore allows users with global.admin permissions to suppress the footer.
Merge pull request #340 from phihag/optional-massmessage-signature

Make massmessage footer optional
pep8
Merge branch 'develop' of https://github.com/liqd/adhocracy into develop
Test for and support umlauts in export (Fixes hhucn/adhocracy.hhu_theme#252)
Generate welcome link in user profile

Often, an administrator wants to assist a user who forgot their password, or has just been imported in. Add a simple button in the user details, where global administrators can (if welcome is enabled) generate a welcome link for users.
Fix default boolean value in migration script for MySQL (Fixes #hhucn/adhocracy.hhu_theme#250)
Extend instance import/export (hhucn/adhocracy.hhu_theme#224)

Add new properties and tests for import/export
Log errors in link creation
Pass on kwargs when generating comment URLs (hhucn/adhocracy.hhu_theme#65)
Always use absolute URLs for event links (Closes hhucn/adhocracy.hhu_theme#65)
Use something slightly less crazy to enumerate notification types
Merge pull request #342 from phihag/welcome-link-in-user-profile

Generate welcome link in user profile
Merge branch 'develop' into welcome_back

Conflicts:
	src/adhocracy/lib/message.py
Merge remote-tracking branch 'liqd/develop' into welcome_back

Conflicts:
	src/adhocracy/controllers/user.py
	src/adhocracy/lib/auth/welcome.py
Fix syntax error
[welcome] Do not require password to be unset, and support the case of both welcome_code and reset_code