search: fix anchor ids for duplicate headers

[tommilligan]

Closes: #720, #1579

Currently, if one page contains two sections with the same header, the generated anchor IDs will be uniquely enumerated:

<h3 id="my-title">
  <a class="header" href="#my-title">My Title</a>
</h3>
...
<h3 id="my-title-1">
  <a class="header" href="#my-title-1">My Title</a>
</h3>

However, search doesn't match the way it generates ids from header content, and doesn't enumerate them. This leads to all search results for a given section header, linking to the first section header with that content on the page.

---

This PR factors out unique id generation for headers to a new function, unique_id_from_content. The existing function id_from_content is publicly exported and so cannot be deleted - it has been marked as deprecated from the next patch version 0.4.16.

Matching tests have been added, and I have verified that this works for my specific use case locally.

Let me know if there's anything that needs changing - thanks very much for maintaining this project!

[ehuss]

Very nice, thanks!