Retry deletion of message if deletion fails

[BepisGithub]

Bug being solved:
Prior to these changes, if deleting a message threw an error, the script would stop deleting any more messages. This was inconvenient because on slower connections a deletion may time out from time to time and throw an error, which would cause the script to stop.

Changes:

• Now the script tries to delete the message 5 times with an adjustable delay in-between attempts. If after 5 attempts the deletion fails, the script moves onto the next message.
• An option was added to the UI to adjust the retry delay. Meaning that if deleting a message throws an error, you can chose how long the program will wait before attempting to delete it again.

Implementation:

I wrapped the code to delete a message in a do while block and used two variables to ensure the block would get run again if there was an error after trying to delete the message (for a maximum of 5 reattempts). If there was no error, the script would move on to the next message.

To ensure the script would continue working if there was an error when deleting the message, I nested the "if (!resp.ok)" statement within an if statement that wouldn't evaluate to true if resp was undefined (this fixed the script stopping if deleting the message threw an error).

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This change is 

[BepisGithub]

This closes #132