Add toggle to hide slopes in alarms

[crsohr]

This PR adds a new toggle in the Settings to opt-out of seeing slopes in alarm notifications. This setting is enabled by default (preserving the functionality of showing the slope by default).

Rationale: I am using FL2, and slopes are my main source of anxiety in the official app, as I find they are seldom accurate, and seeing “↓↓” on a “low alarm” doesn't help when it's just the sensor being sloppy and incorrect.

[paulplant]

Hi @crsohr , I was looking at this PR the other day and it's a tricky one to include.

The problem is that the notification should show the trend arrow. Adding yet another option which we could argue is just a "personal preference" for just one person who wants it is a bit difficult and we really try to always avoid PRs that are forcing personal preferences of just one person into the app.

I've been using a Libre 3 for the last 12 days whilst testing the heartbeat function and the L3 (via LibreLinkUp) is shockingly noisy, so I understand why the trend arrow should not be really relied upon in this case, but surely the solution should be to maybe add smoothing to the downloaded values to make them less erratic. @JohanDegraeve , what do you think?