Improve visual progress when sending message with attachments

[tomholub]

Right now, when sending a message with attachments, it seems to get stuck for a long time and it's not clear if it's progressing or not.

We could add a progress title to the loader? It would say:

• encrypting (while it's spending time in Core.composeEmail)
• uploading x% (while it's spending time with Gmail API)

[sosnovsky]

@tomholub I also noticed a big jump in memory usage when sending mail with attachment - from 100mb to 560mb, and it stays even after mail is sent

Screen.Recording.2021-10-22.at.14.34.49.mov

[tomholub]

I think the memory should eventually get freed. Nevertheless, one day we'll implement streaming encryption, and that will take care of that.