Adjust available routes based on theme presence

Themes do not necessarily want all of the available LMS routes, such
as `/jobs` and `/university_profiles`. This change splits up the
`lms/urls.py` file and selectively enables/disables routes based on
whether or not a theme is enabled. This is a naive solution for now;
a better solution gives themes a way to selectively overrides such
routes.

Additionally, with the `MKTG_URL_LINK_MAP` setting that hits certain
routes immediately on each page render (whenever the `marketing_link`
helper function is called), themes may crash if they don't leave
all marketing link routes present in `lms/urls.py`. This change also
provides the ability to override the `MKTG_URL_LINK_MAP` in the
settings.

Finally, modify the mitxmako marketing URL middleware to not try to
reverse disabled URLs, which are those keys in the map whose values
are `None`.
Use default `PLATFORM_NAME` in AWS settings

Rather than require the `PLATFORM_NAME` to be specified in the
`ENV_TOKENS`, allow the default setting in `common.py` to be used.
Change `if ... is not None` to `if ...`

Minor stylistic tweak.
SASS diff from stanford themed branch for master
fix a few text weights and moved ugly mixins to mixin file and implemented in shame file
edx.org - revises new register/login banner image variable paths to resolve a Sass rendering issue
theming - resolves some variable defintions to help with edX theme
theming - revised variables and link/button styling to account for edX's registration and login constrast/styling changes