Improvement the way how Issue fields are loaded by using a lazy load mechanism

The mais goal of this improvement is to reduce the memory usage at runtime and improve the Xporter performance.

This story is composed by 2 parts:

1. When a template is created/updated, all fields should be extracted and saved on database.
2. Fieldmapper should receive these fields and must load only these fields when the mapping is being created.

Note: It's necessary to create a migration task to extract all fields from templates already installed.