Fix constants in templates

[tmaybe]

• [x] validate approach of using config.py to hold constants
• [x] apply solution to remaining constants that can be handled this way
• [x] add test coverage

Closes #440

[tmaybe]

This is ready to merge when the Travis CI builds pass.

I set up the system and added all the constants that appear in templates. There's room to add more constants, but I'll leave that up to future discretion.

[migurski]

:+1: