Create localization file for webadmin application

Added translation tags to webadmin application and created blank localization file.

To customize strings in the interface:
* `cd components/tools/OmeroWeb/omeroweb/webadmin`
* `python ../manage.py makemessages -l en`
* edit `components/tools/OmeroWeb/omeroweb/webadmin/locale/en/LC_MESSAGES/django.po`
* `python ../manage.py compilemessages`

This will create `locale/en/LC_MESSAGES/django.mo`.

Not sure how this integrates the build process.