- Discount excluded pages

- Update the section on configuration of collections to include normal pages.

- Avoid irrelevant conflicts when updating from master

- Remove duplicated configuration code from `_tests_config.yml`
- Rename to `_tests.yml`
The tests collection is ignored when not mentioned, so it does not need to be excluded in `_config.yml`.

- Fix a bug with the breadcrumbs list, where a link on a grandchild page can point to the wrong parent page, following the solution suggested by @AdityaTiwari2102 in just-the-docs#477

The TOC/breadcrumb links in "Tests" and "Test Collection 1" exhibit the bug reported in just-the-docs#492. To fix the bug, this PR needs merging with just-the-docs#494.

Serve locally:
```
bundle exec jekyll serve --config _config.yml,_config_dev.yml
```
Test locally:
```
bundle exec jekyll serve --config _config.yml,_config_dev.yml,_tests.yml
```

- Fix a bug that affects sites with no child pages

- Relax version constraints in response to failing check
  `Could not find gem 'bundler (~> 2.1.4)' in any of the relevant sources`

- Cover all combinations of `search_exclude` and `nav_exclude` in `_tests.yml` Searching for C2 and C4  should return results, C1 and C5 not.
Navigation for pages, C1, C2, C3 should appear, C4 and C5 not.

Manually merge the relevant changes from PR just-the-docs#494, namely:

When configured for more than one collection, v0.3.2 produces empty TOCs of children in all but the last collection, due to referencing the overwritten `pages_list` in `_layouts/default.html`. When the same parent title is used in different collections, v0.3.2 can also produce a link to the wrong collection. Moreover, v0.3.3  can also produce incorrect breadcrumb links; see issue just-the-docs#492.

- Move the breadcrumbs link variable assignments from `_layouts/default.html` to `_includes/nav.html`, and make it conditional on `page` being in the current collection.

- Correct the condition for assignment to `second_level_url`, as in just-the-docs#477.

- Change the variable used for the TOC from `children_list` to `toc_list`, move its assignment from `_layouts/default.html` to `_includes/nav.html`, and make it conditional on `page` being in the current collection.

Setting `nav_fold: true` for a `just_the_docs` collection makes it appear like a folded top-level navigation element.
Its navigation is unfolded when any page in the collection is active.