Add custom trigger hashtag for each client display

[palpen]

At present, each client is checking Twitter for a text prompt using the same hashtag. A new tweet with the trigger hashtag (currently, #dalle) will update the image in all client displays. This issue will make the changes that will assign a unique trigger hashtag to each client so that they can be controlled from Twitter independently (but also includes the option to change them all simultaneously).

Implementation:

• Move the trigger hashtag from tweets_utils.py to the config yaml files
• Add hashtag that will change all displays (probably just keep the default, #dalle)
• Update retrieve_tweets_containing_text_prompt function in tweets_utils.py to include an argument for each trigger hashtag

[palpen]

Need to address this this issue before proceeding to avoid a hacky solution