This function takes in a set of groups, and their consituent
channels and writes the appropriate .mbsyncrc block. The block is as
shown below:
Group groupName1
Channel channelName1
Channel channelName2
Group groupName2
Channel channelName3
Each group must have a unique name, no matter which account it is
declared under. The same holds true for channels. However, if there is
a group that shares the same name as the channel, the channel will
effectively be "shadowed" by the group, and mbsync will default to
working with the group in that case.
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