This function takes in a set of groups, which includes their
consituent channels and writes the appropriate .mbsyncrc block for the
channel. The block that is generated is shown below:
Channel groupName1-channelName1
Master :<accountName>-remote:<master-pattern>
Slave :<accountName>-local:<slave-pattern>
Channel groupName2-channelName2
Master :<accountName>-remote:<master-pattern>
Slave :<accountName>-local:<slave-pattern>
Each group must have a unique name, no matter which account it is
declared under. The same holds true for channels.
Using channels with the patterns set up this way allows one to specify
which maildir directories are to be synchronized FROM the master TO
the slave. In addition, it allows for these maildirs to be remapped,
between the master server and the local slave.
This is critical, because Gmail has a strange way of storing its mail
that makes using mbsync, mu, and mu4e more difficult.
There are additional channel parameters that are already present in
this codebase from the previous use of group-channel configuration,
which will be reused.
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