Add support for Pinning and Unpinning messages

[okzyrox]

Adds support for pinning and unpinning messages from the message context menu

• Also includes a fix to the "Delete" message context menu option that showed up for other recipents messages despite the user not being the author for them when in DMs/Group DMs
• And an addendum fix for the previous PR I submitted with the permissions for Pinning/Editing messages when it is checked on click (curse Visual Studio 2022...)

Note:

• Ideally we'd use the new PIN_MESSAGES permission for this, however it hasn't been fully seperated from MANAGE_MESSAGES yet (as DMs/Group DMs seem to still use it instead of PIN_MESSAGES....) and given that the check for showing the "Pin Message" menu item also uses MANAGE_MESSAGES as a condition, I decided to just use the old permission.

[iProgramMC]

Thank you!