Merge tag 'v1.8' into development

Release 1.8
Show only active tasks in the overview view

Active tasks are unbilled tasks that are tasks that are either fixed cost
or that have time entries that should be billed.
Show a total of unbilled time and earned money (excl. VAT) per customer
Build a span per customer so that they flow and use two columns in the overview
Ignore style.css.map generated by Sass
Merge branch 'feature/02ad0e-improve-main-overview' into development (closes: #02ad0e)
Ignore generated YARD documentation and cache
Rework all documentation in YARD format
Merge branch 'feature/e1ac56-yard-documentation' into development (closes: #e1ac56)
Remove a very old mockup
Add a Rakefile for the 'yard' documentation task
Remove the TODO file

All TODO entries were either already fixed or are present as ticgit
tickets.
Show only active tasks in the invoice select form

If there are tasks with an hourly rate that have no time entries,
then it's not worth showing them.
Add in_current_month? method to the TimeEntry model
Only select time entries that are not in the current month
Merge branch 'feature/ff83ce-preselect-time-entries-invoice-form' into development (closes: #ff83ce)
Bump version to 1.10
Update the changelog for the new release
Merge branch 'release/1.10'