cylc gui: fix handling of invalid regexes for filters
unset filtering on bad regex in dot view
Allow calendar mode string to be flexible

isodatetime update: d4dd6d95adc9a46f892067f2c99e0e510bb9d9d1.
Remove hard coded calendar type string

Use constants in `isodatetime.data.Calendar` instead.
Merge pull request #1017 from benfitzpatrick/cylc-gui.fix-bad-regex-handling

cylc gui: fix handling of invalid regexes for filters
Merge branch 'master' into 119.iso8601-cycling
Fix import statement
Merge pull request #1022 from matthewrmshin/flexible-calendar-mode

Allow calendar mode string to be flexible
#983: use sequence-generated cycle points for runahead calculation
#983: update and fix conflicts