Drop django-maintenancemode

We have decided against it as it has not been updated in a while.
Users who wish to use maintenancemode can easily install and add it
in their settings.
Docs: imported upstream doc to build upon
Templates: allowed inheriting and overriding templates

By using the django-overextends package templates can be inherited AND
overriden. This enables block-level customization with no need to
copy-paste the whole template. In order to use this behavior, just place
`{% overextends 'original_template.html' %}` at the beginning of any
custom templates.

Added extra docs for customizing static assets too.

Fixes #257.
Templates: externalize logo, bring back stock graphics

Any customizations must override the `logo` block.

Fixes #258.
Templates: removed contact form blurb

This can be customized in the `contact_preamble` block.

Fixes #259.
Templates: implemented generic about page

Brings back stock Pootle content.

Fixes #262.
Templates: externalize custom header/footer content

These can be customized in the following blocks:
  - `nav_items_pre`, `nav_items_post` for navbar items
  - `footer_items_pre`, `footer_items_post` for footer items

Fixes #261.
Cleanup: removed obsolete templates
Templates: generic welcome page and bg

Fixes #260 and fixes #264.
Templates: bring back stock Pootle login page

Registration has been taken out for now since that will need to be handled
separately.

Fixes #263.
Cleanup: use Django's own staticfiles' template tag

No point in duplicating functionality here.
Templates: removed references to custom fonts.

Fixes #266.
Docs: mention that virtualenv needs to be enabled
Docs: added reference to make assets
Docs: dropped favicon section

It's not different from any other asset.
Cleanup: removed latest users section from admin

The latest users are being displayed as part of the users page, there's no
need to duplicate functionality here.