less: use .label and .label-* style instead of custom .*tag

This reduces the code and makes it more consistent.

Visualy this results in bold texts (which in case of small fonts is easier to read)
and a bit more padding left and right.

For a proper spacing, this requires to remove whitespaces before and after
the texts inside the labels.

Since the name changed from tag to label, also rename the variables and functions.