[FEATURE]: Support replacement of event titles with icons/emojies

[aceisace]

Extend the calendar and agenda module so that given strings are replaced with icons/emojies.

To-dos:
[ ] Consideration of font to support icons/emojies
[ ] Update web-ui to support replacement of strings in event titles of agenda and calendar module. Find a way to add more fields in case several strings should be replaced
[ ] Update code
[ ] unittests
[ ] Create PR and merge after all tests have passed