Initial support for saving files using Slack's file url

[jcpsantiago]

Some messages will have a file, or files, attached. These are important for context, and should be stored together with the message text. Uploading the files to Confluence is an annoying process, plus we may have space or file size constraints. The ideal scenario is to store the text in Confluence and some other storage e.g. Google Drive/S3 together with any connected files. In situations where we cannot save the files in such remote storage, we should at least keep a link to the file in Slack.

Still need to investigate:

• how to determine if a message has a file attached
• what is the lifetime for files in Slack's storage
• whether the file URLs in Slack are stable permalinks

Afterwards, we should implement:

• Every message containing a file attachment is stored in Confluence in the format:

Timestamp
Name
---
Message
---
Attachments:
- file1
- file2