Fix link to website

See merge request !4

see <https://blog.sparksuite.com/7-ways-to-speed-up-gitlab-ci-cd-times-29f60aab69f9>

CI: cache only downloads

See merge request firmware/site-ffs!3

CI: explicitly specify what to cache

See merge request firmware/site-ffs!2

CI: take GLUON_BRANCH from build environment if specified

See merge request firmware/site-ffs!1

CI: add cache for builds

See merge request freifunk-region/stuttgart/site!4

Build cache is kept per build target. Each tag also has a seperate build
cache, so tagged builds are always built from scratch.

CI: set GLUON_BRANCH depending on build type

See merge request freifunk-region/stuttgart/site!3

Introduce the following build type:
| Build Type | When   |
|------------|--------|
| stable     | commit tag starts with "stable/" |
| beta       | commit tag starts with "beta/" |
| nightly    | trigger is a scheduled job |
| commit     | else |

Depending on the build type, set the GLUON_BRANCH accordingly:

| Build Type | GLUON_BRANCH  |
|------------|---------------|
| commit     | (unset)       |
| nightly    | nightly       |
| beta       | beta          |
| stable     | stable        |

CI: do not build all targets on all branches

See merge request freifunk-region/stuttgart/site!2

When building non-tagged and non-scheduled commits, only build
x86-64 to speed up the build.

create manifest after building

See merge request freifunk-region/stuttgart/site!1

This makes the gluon build fail since v2018.2.1

allow the SSID to change to the offline SSID once every 15 minutes