Add support for downloading sentiment data

[jaARke]

Users might want to manually sift through some of the news/social media data we retrieve about a specific stock. To allow for this, we should add another command to cogs/sentiment_cog.py, allowing the user to query for sentiment data and download that data in CSV format. This will require the following (in a new command function within the sentiment cog):

• Calling backend functions to retrieve the list of data to be saved
• Formatting data in CSV format -> the user should be able to discern if a given data point is a headline, a tweet, or a Reddit comment.
• Uploading the CSV file to Discord

Note that this command doesn't require any data analysis. It is just a way for the user to interface with our data retrieval system.

[jaARke]

This kind of functionality will need to be handled in the discord-lambda-py template project before our project can leverage it.

I will work on adding the functionality and add an update here when it's ready to be ported into this project.