Rename competency levels from "x to y" to y (#352)

* rename junior-to-mid to engineer

* rename mid-to-senior1 to senior1

* rename senior1-to-senior2 to senior2

* rename senior2-to-principal to principal

* update level titles

* update documentation about levels

* fix aria-current on How To Use page link

* fix reference to old levels

* update competency.yml comments

* add redirects from old level urls

* generically build all yaml files in the data directory

* get api version from data file not circle tag

* have version endpoint still return circle tag

* gracefully fail if circle tag is missing

* bump api version to v2

* fix remaining reference in api docs

also update some formatting

data/api_version.yml

@@ -0,0 +1 @@
+version: 2.0.0