Modify altimetry logic, if there is no AGL/AMSL data in the airspace
file, assume that it is AMSL, as that is what ICAO/ATC would do.
Merge pull request #179 from TobiasLohner/airspace

import_airspace: orient all airspaces clockwise
mapserver: update airspace files
Revert "flights: Use subqueryload() instead of joining everything in one huge query"

This reverts commit a193934410cffca56e1d03c80f688af6bffbfa03.

That did *not* work!!
mapproxy: serve 8-bit PNG files

Reduces file size by over 2/3 with minimal visual impact.

Requires MapProxy 1.6 with Pillow for image processing for suitable
results.
forms/club: Added Optional() validator to website field

The URL may also be empty if e.g. just the name is changed.
Merge pull request #178 from shadowoneau/lj-altdefault

Modify altimetry logic in import script
Merge pull request #181 from Turbo87/optional-url

forms/club: Added Optional() validator to website field
assets: Updated bootstrap to v3.0.1
templates: Disabled IE compatibility modes
templates: Use external Respond.js polyfill on IE8 and below

.. as suggested by bootstrap.
assets: Converted respond.js to bundle
Merge pull request #182 from Turbo87/bootstrap

Updated Bootstrap library
Assets: use local copy of ubuntu font
Merge pull request #183 from TobiasLohner/font

Assets: use local copy of ubuntu font
i18n: Updated translation template
i18n: Updated German translation
Merge pull request #184 from Turbo87/i18n

Updated translations and template
model: Added tracking_callsign column to User model
tracking: Don't show "other pilots" table if it's empty