Ensure meal is only saved once

[MikePlante1]

• Disables the Save and continue button after it's pressed until the AddCarbs module is reopened.
• Renames saved flag to noteSaved for clarity.
• Addresses #175

[aug0211]

• Multiple days of testing in vitro and in vivo, 0 issues encountered
• Button grays out as expected immediately on tap
• Attempted to recreate instigating issue a number of times in vivo with a looping pump (and CGM) connected by entering carbs during loop cycle
• 0 instances resulted in duplicate carbs, despite my best efforts to break it
• 0 other issues observed over the course of multiple meal entries (both in vitro and in vivo)

[dnzxy]

Merging with 2 testers (in-vitro and in-vivo) and 3 approvals.

[MikePlante1]

From the code, I'm pretty confident the button won't be able to be pressed twice, but I've still been spamming the "save" button every time I log meals the past few days and haven't had it trigger twice, even when purposely entering during a loop cycle.