Add macro to play All Star chorus and print lyrics, fixes issues qmk#…

…3345, qmk#3346, qmk#3347, qmk#3348, qmk#3349, qmk#3350 (qmk#3372)

Instruct macos users to use avr-gcc@7 (qmk#3337)

If you run `brew install avr-gcc`, you get a version that has
compatibility issues with LUFA. I updated the getting started guide for
osx, the qmk_install setup script, and added a section to the FAQ for
folks like me who accidentally updated avr-gcc past 7.

Merge pull request qmk#3229 from qmk/hf/shinydox

Adds I2C timeout and return values, adds support for future RGB Ergodox EZ

Addition of hard brigtness limit for RGB_Matrix (qmk#3299)

* Addition of hard brigtness limit for RGB_Matrix

- Added a define "RGB_MATRIX_MAXIMUM_BRIGHTNESS" to enable hard limiting the maximum brightness for rgb_matrix
- Used the above define to limit the maximum brigthness of HS60 for better stability

* Added docs for new rgb_matrix define

* Addition of check for maximum brightness

Diagonal micro move fix

Diagonal micro move fix

removed unused boxes (qmk won't compile with them anyway), updated we…

…lcome message

Clean up SEND_STRING keycodes and add media keys (qmk#3214)

* Clean up SEND_STRING keycodes and add media keys

* Remove stray define

* Add missing SEND_STRING keycodes for completeness

Also, add KC_EJCT to the keycode references

Disable JTAG in keyboard init

So that all boards, present and future will have JTAG disabled, even if using a custom matrix

Disable JTAG in keyboard init

So that all boards, present and future will have JTAG disabled, even if using a custom matrix