Convert resume pdfs to images

[mcpenguin]

Summary of Changes

PDFs sent to the #resumes channel will be converted into images automatically by Codey.

TODOs Before Merging

• We need to update the staging and production dockerfiles to include these lines that download deps necessary for the PDF->image converter:

RUN apk add --update ghostscript
RUN apk add --update graphicsmagick

• We need to update the staging and production configs to account for the new config variable, RESUME_CHANNEL_ID (see below)

Testing Notes

1. Add the RESUME_CHANNEL_ID variable to your config.json; this is the channel where the resumes are sent (in this case it will be #resumes)
2. Send a message without any attachment: nothing should happen (this is similar to when people post google drive links to their resumes)
3. Send a message with a pdf of a resume: Codey should automatically send back an image corresponding to the resume.

Screenshots

Nothing happens if no attachment is sent, so the screenshot for the case where a resume pdf is attached is shown below:

[alexz429]

Some small optimizations for dockerfiles:
Add the run steps for mkdir and library installations at the very start, since those actions can probably be cached before any file changes.
Add /tmp to dockerignore to prevent local tmp files being sent over.

Also don't forget to add the new env var into staging and prod configs.