Replace moment-timezone

[benbucksch]

Unfortunately, node-ical depends on moment-timezone, which in turn depends on the entire moment library, which its own developers (!) say is too huge and difficult to "tree-shake" (reduce). Which in turn means that our app is unnecessarily slower to load.

There are only about 5 uses of moment in node-ical. 2 of them to get the local timezone, a few to parse the timezone, and 1 to strip the time from a date. These should be relatively easy to replace.

Tasks:

• Find a library for timezones. See whether Intl is sufficient. Make sure the timezones used in ICS and my Microsoft Outlook (Windows timezone names) are supported.
• Modify node-ical source and replace moment with the above lib.
• Test it.

[benbucksch]

Downstream issue: mustang-im/mustang#267