Add separate intervals for checking progress and posting messages

[pietroalbini]

The current implementation of the !deploy_app command checks every n seconds whether the build is finished or not, and posts a message every time it does so.

crates.io builds are really slow (taking around 20 minutes each), so posting a message every iteration spams the channel too much. If we increase the interval for posting messages though we risk delaying the finish of the build, as the bot would wait the same interval even for checking whether the build finished or not.

We should implement two separate intervals for this.