replacing gist files with data folder files

I made few changes to use sample files from data folder instead of gist URL's.

- What is done:
   - Defined data filenames as  constants under `constant.py`
   - Defined `get_data_file` helper function under `helper.py`
   - Modified just one test `test_create_template` under  `tests/ui/test_template.py`

- What is pending 
  - Need to remove all URL's from constant.py
  - Need to update all tests which are still using URL along with import statements

I'll work on pending part once the changes related to data dir will be accepted.