Added new fields and styling for Campaign pages to allow for CTA-first layouts

[mtdenton]

Closes #1755

This adds a few things to campaign pages:

• cta_first boolean that determines the order of the markup
• When cta_first is checked, it retains the old layout style on tablet sizes and above
• Replaces hard-coded "Donate now" text with an editable intro section header
• sections of campaign_page.html have now been split out into fragments to increase readability

Checklist

Remove unnecessary checks

Tests

• [ ] Is the code I'm adding covered by tests?

Changes in Models:

• [ ] Did I update or add new fake data?
• [ ] Did I squash my migration?
• [ ] Are my changes backward-compatible. If not, did I schedule a deploy with the rest of the team?

Documentation:

• [ ] Is my code documented?
• [ ] Did I update the documentation?

[mtdenton]

Thanks for coming up with a solution with better A11Y support. In addition to the inline comments I left, I also have a question regarding if a new form header is needed. Having two big headers on the same viewport seems to make the page a bit busy. Maybe this is something we should check with the design team to get styling suggestions.

@mmmavis Happy to pull out donate_header, just need to find another solution for getting the intro_header above the CTA on mobile that doesn't duplicate that h2 in the HTML. I'm thinking the best route is to pull it out into its own row, so I'm going to play around with that a bit.

[mmmavis]

Thank you @mtdenton for making the updates.

One last thing -

I noticed that when cta_first is off, we see two headers on narrow screens. Once that's fixed, this PR is good to go!