PR: Add Travis CI building and deployment to theme example site

[CAM-Gerlach]

This PR sets up the example site to build and deploy automatically to Github pages using Travis CI, as well as check that every PR at least builds successfully.

To complete the process, @ccordoba12 I either need admin on this repo, or you need to:

1 Activate Travis CI for this repo
2. Add your environment variables LEKTOR_DEPLOY_USERNAME and LEKTOR_DEPLOY_PASSWORD with your username and a personal access token (or a bot's, etc) in the Travis settings there
3. Trigger a build on this PR and make sure it works
4. Activate Github pages for this repo in the Github repo settings
5. Activate branch protection on master and gh-pages, restrict push access to both to admins and e.g. members of the website team (which you should presumably add me to :) ) and require reviews and status checks on master.

Finally, this can be merged, and the demo site should go live.

[ccordoba12]

To complete the process, @ccordoba12 I either need admin on this repo

You should be an admin now.

[CAM-Gerlach]

Thanks @ccordoba12 ; Travis and Github Pages are now good to go, branch protection is set up (for master and gh-pages; I left legacy as it is for now), the description, link and tags are updated, and the demo site is live.

Now I just have to add support for mainpage flowblock nesting and add an option to make Services/Author tiles clickable links, which will allow for easily re-implementing the Sponsors section site-side with most of the fixes/enhancements I proposed in the original PR, and then I can implement it on Spyder-Website itself for your review.

I can also submit it to the Lektor-Themes repo and the Lektor website gallery at this point, though I may as well do those two things first to ensure we can properly show it off on our site as well as mine and the demo one.

[ccordoba12]

Ok, thanks! It looks really nice!