Fix template checking order in site.go

- Change order of HasPrefix to match correct order
- Remove theme concatenation to _internal in last loop of
  appendthemetemplates so it looks in the right place for internal
templates
Remove extraneous printlns used for debugging. Fixes #635
Restore main XML feed to index.xml
Remove redundant check for DisableRSS
Merge pull request #638 from k4rtik/mainrss

Restore main XML feed to index.xml

* k4rtik/mainrss:
  Remove redundant check for DisableRSS
  Restore main XML feed to index.xml
Merge pull request #637 from realchaseadams/bugfix-635

Fixes #635

* realchaseadams/bugfix-635:
  Remove extraneous printlns used for debugging. Fixes #635
  Fix template checking order in site.go