Make UI strings customizable more easily

[aaaaalbert]

The current custominstallerbuilder HTML has lots of mentions of "Seattle", Seattle-specific terminology, and "Custom Installer Builder". These are hardcoded which makes it hard for people forking the custominstallerbuilder to, e.g., use a different testbed name than we do.

The obvious fix is to replace said instances by constants that will be substituted by Django using the {{ CONSTANT_NAME }} syntax in the HTML code, and defining these constants in local_template/settings.py. (Grep the sources for MEDIA_URL to get an idea how this approach works).

(See also SeattleTestbed/clearinghouse#139)

[aaaaalbert]

@asm582, want to have a shot?

[asm582]

Ok i will work on this...

[asm582]

Found changes in the below files:-
local_template/settings.py
settings_base.pyc
html/templates/base.html
html/templates/download_keys.html
html/templates/download_installers.html
html/templates/builder.html

Since our DJANGO_SETTINGS_MODULE is local.settings so i think DJANGO expects the constants to be placed under custominstallerbuilder/local/settings.py and i have initialized context_processor.py from settings_base.py.

[aaaaalbert]

@lukpueh...?