Fix link to contributing guide

The link was pointing to a not very helpful page in the x-api repo.
It's now pointing to the contributing guide in the docs repo.

kindergarten-garden: clarify test names

For the custom garden tests it wasn't very clear that the students will
be sorted in alphabetial order before being assinged their plants. This
has been added to the test names to make it more clear.

MAINTAINING.md: add link to docs repo

Remove reference to project board

We haven't been using it to set priorities for a while now.

Remove link to dead todo repository

Replace references to repo name `xjava` with references to repo name `java`

all-your-base: update to v1.1.0 of tests

Remove leading 'x' from repository name

The leading 'x' is kind of arbitrary. Especially now that we can set
topics on the repositories, we don't need a pattern to distinguish what
is a track or not.

The repository itself has already been renamed. GitHub redirects from
the old name to the new name, so we do not have to rush to fix links to
the old repository name, though we should update them for the sake of
clarity.

I chose to skip the x-common test case "both bases are negative" since
the xjava implementation of this exercise forces the first base to be
validated before a second base can be supplied. This test would
therefore be redundant since we already have tests that validate the
first base.

Removing final from starter classes

Add policy for 'final' classes

run-length-encoding: estimate difficulty 5