added topics for word search

added topics for the queen-attack problem

Added @FridaTveit suggestions to nucleotide-count topics

* Updated Food-Chain Topics :)

* Removing plus signs from tags for food-chain

* minus comma

Added four tags to nucleotide-count topics as described in #808.

* issue fixed

* minor bug fixed

* issue fixed #789

* minor bug fixed

Referencing #788

conditionals: finding if a char is repeated
loops: looping through a string's characters
parsing: parsing a string for repeated characters
strings: self-explanatory :)

referencing #788

conditionals: finding if a char is repeated
loops: looping through a string's characters
parsing: parsing a string for repeated characters
strings: self-explanatory :)

Fixes #782

Reference solution directory was renamed and move to the
always-excluded-from-delivery .meta folder.

Side exercises can only be unlocked by core exercises. Previously we
had exercises being unlocked by both core and side exercises.

Each core exercise should unlock at least one exercise. Previously we
had some core exercises that didn't unlock any side exercises. Now
that's only true of the last core exercise in the track.

Update config.json to set which exercises are core and which ones are
unlocked by other exercises.

The slug is not used anywhere. We initialize a track based on knowing the Track ID.
Since the repository is always named after the track ID, this field, too, is redundant,
as it can be inferred.