DEV: Migrate settings from deprecated icon names (#37)

* DEV: add migration to update from deprecated icon names in header_links settings

* DEV: add migration of deprecated icon names to svg_icons site setting, also rename setting to lowercase of svg_icons

* update discourse-compatibility

Author: tyb-talks

.discourse-compatibility

@@ -1,3 +1,4 @@
+< 3.4.0.beta2-dev: 0b7a567952514f1e0a0fe7b73d2851925a87b018
 < 3.4.0.beta1-dev: 95a6c3d117191353de8738890728e10cc560cefe
 < 3.3.0.beta1-dev: 064fa004f98f71e24f2994a4090973eb1eec7523
 3.1.999: fe4c1be3e451a053a1e42895b7479fc560622b47