DiscordBot improvements

[SmithsonianDSP]

• [x] Improve OC stream announcing. It occasionally seems to find and announce the same OC post twice, but usually an hour or so apart. (Currently it is using a place_holder parameter to set the end of where praw should retrieve posts up to, but the praw docs mention this can be unreliable if the post ID used as the placeholder is removed. To prevent this, also store the ID of the last OC post that announced and break the loop if reaching a submission matching that or the placeholder ID is hit.)
• [x] For inbox feed: determine if an inbox comment reply is a reply to the post or a reply to the source/attribution comment and distinguish in the Discord message.
• [ ] For inbox feed: determine and distinguish username mentions from other comments/replies
• [x] Make the various limit=num values dependent on what the RETRY_MIN global is set to, so that can be adjusted without potentially having the DiscordBot grabbing too many or not enough items from the streams.
• [x] Send certain logging/info messages to the #discord-bot channel (?)
• [x] Add update messages for GitHub IssueCommentsEvents when somebody comments on an issue
• [x] Expand support for IssueEvents when issue labels or assigned persons are changed or issues are edited/updated.
• [x] Implement support for announcing GitHub PullRequest events.
• [x] Update setup.py to include new requires (discord.py >= v0.10, github3.py)
• [x] Accept passing of flags/parameters to specify different behavior or disable different functions
• [ ] Additional exception catching in sub-methods
• [x] Refactor out formatting functions to separate module for cleanliness
• [x] Update GitHub3.py to latest version (inludes many breaking changes)

[SmithsonianDSP]

Test relaying of issue comments to Discord

[amici-ursi]

marked off GH events because they are handled natively with a GH-Discord webhook