Set up the CTA for the lab page and make the fund CTA work with labs

opentech/apply/funds/migrations/00014_add_meta_names.py

opentech/apply/funds/models.py

opentech/public/funds/migrations/005_link_text_optional.py

opentech/public/funds/models.py

opentech/public/funds/templates/public_funds/fund_page.html

...funds/templates/public_funds/includes/fund_apply_cta.html

.../funds/templates/public_funds/includes/lab_apply_cta.html

opentech/public/funds/templates/public_funds/lab_page.html

opentech/public/utils/templatetags/util_tags.py