Convert all string references from const char* to String/F() to store values in flash instead of ram.
Memory usage from `main`:
```
Sketch uses 27878 bytes (90%) of program storage space. Maximum is 30720 bytes.
Global variables use 1755 bytes (85%) of dynamic memory, leaving 293 bytes for local variables. Maximum is 2048 bytes.
```
Memory usage after these changes:
```
Sketch uses 28054 bytes (91%) of program storage space. Maximum is 30720 bytes.
Global variables use 1445 bytes (70%) of dynamic memory, leaving 603 bytes for local variables. Maximum is 2048 bytes.
```
This provides a dynamic memory savings of 310 bytes!
Reviewed-on: https://git.pinkduck.xyz/adam/libGravity/pulls/8
Co-authored-by: Adam Wonak <adam.wonak@gmail.com>
Co-committed-by: Adam Wonak <adam.wonak@gmail.com>
Adam Wonak
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