The gitter mentions are in the format '@usermention'
and the mentions are included in the export data as:
"mentions": [
{
"screenName": "usermention",
"userId": "54d7876c15522ed4b3dbbefb",
"userIds": []
}]
We extract this data and map this mention to @**usermention**
for Zulip.
Messages can be bulky, and storing them in a single
data structure can cause a memory error.
In this commit, the messages are written to a file
batch-wise, thus avoiding the memory error.
Similar to commit 6b7b6b38ad
