Disable files and images on v1

[jgaehring]

In response to #444, this PR will do 2 things:

1. Remove the files field from all logs.
2. Disable the ability to add images to any logs that have already been synced to the server. The images field will be stripped off logs before sending them to the server if a server-assigned id is present on the log.

Number 2 introduces a significant restriction of the images feature, but I think this is necessary until we overhaul images (#45), which will probably only be implemented in version 2.x. So this will be a permanent limitation in 1.x. Included in this PR is an explanation of why we're restricting this feature, which will be displayed on logs that have already been synced.

[mstenta]

Tested this and it seems to fix the issue! I tested three scenarios:

• Creating a log on the server with a file, pull into Field Kit, edit the Notes field, sync back - the file is untouched.
• Creating a log on the server with a photo, same as above - file is untouched.
• Creating a log in Field Kit with a photo - it syncs properly, and re-editing it does not touch the photo.