1961-welcome-accessibility-2

See ticket: https://tahoe-lafs.org/trac/tahoe-lafs/ticket/1961

This diff replaces the connection indicators which were colored circles to have distinct shapes to improve usability.  This includes changes to unittests although those changes are substring matches against rendered templates, and thus may be brittle (and could conceivably lead to false positives).

I've also tested this briefly by examining the front status page without a connected introducer, with a connected introducer and no known storage servers, and with a connected introducer with connected storage servers.

I *have not* tested the front page display for these cases:

* Known storage servers with a *disconnected* introducer.
* At least one storage server disconnected.