Split /tldr command's support for public sharing into own command

[meetbryce]

Currently the /tldr command supports /tldr public in order to post the channel summary to the channel itself.

This should be deprecated and instead moved to its own /tldr-public command. This is consistent with the thread summary shortcuts, and frees up the additional text interface for use in the implementation of #13 across slash commands with a consistent interface.

This will constitute a minor breaking change, since it will require changes to the app installation for all users, and application code changes for those using OSSAI as a python package.

[meetbryce]

We've instead opted to deprecate the public option. Should anyone reading this wish to have it reintroduced, feel free to open a feature request.