Add {nX} pattern to annotation creation

When entities are annotated, the pattern {nX} with X being a number is
now interpreted in a special way: The whole pattern is replaced by a
number that is incremented for each entity being annotated, starting
from X. If X is omitted, i.e. {n} is used, X is implicitly 1. This pattern can
be used multiple times with different X in an annotation.

Closes acardona/CATMAID#822
Annotations: maintain order of annotated entities in back-end

Especially, when counting patterns ('{nX}') are used, a certain order of
neurons that are annotated is expected. E.g. annotating a list of
skeletons from the selection widget with the pattern '{n}' should
reflect the very same order in annotations with incrementing numbers.

The order of the list passed in as POST parameters is now maintained.
Merge branch 'annotation-naming-patterns'