Fix model chat speakers and personas

[EricMichaelSmith]

Patch description

• When running the model chat task, display the speaker names as "Speaker 1" and "Speaker 2" instead of showing the Turker the name of the model id ("TransformerGenerator", etc.)
  • Change this for both the regular model chat and the image chat variant
• Also, allow the persona strings to be shown on the left-hand pane if your pane's text includes the strings "[persona_string_1]" or "[persona_string_2]"
• Also fix an issue with using run.py to run model image chat: worlds_image_chat.py wasn't being set as the correct words file

Testing steps

Regular model chat

Command:

python parlai/crowdsourcing/tasks/model_chat/run.py \
mephisto.architect.port=3002 \
mephisto.task.maximum_units_per_worker=1000000

Before

After

Model image chat

Command:

python parlai/crowdsourcing/tasks/model_chat/run.py \
mephisto.architect.port=3002 \
conf=example_image_chat \
mephisto.task.maximum_units_per_worker=1000000

Before

After

Testing showing the personas on the left-hand side

Example:

[meganung]

Thanks for the fixes and keeping the tests up to date too!