Strip style elements from snippets for #3146

[sylvieed]

Resolves #3146

This uses Nokogiri to remove the style element and its contents from collections' intro_blocks when they're being shown as a snippet on the Find a Project page, because previously the contents of the style element were showing up instead of the collection's description.

With this PR, there is no HTML in the snippet and ampersands show up correctly.

strip_tags(intro_block) vs. Loofah.fragment(intro_block).text

We were initially using strip_tags to remove HTML tags from the collection's description and shorten it for the Find a Project page. strip_tags automatically removed the style element and its contents, so there was no HTML shown like there is in this issue. However, strip_tags escaped special characters like & to &, which we did not want.
So we switched to Loofah.fragment.text, which gives the option to not encode_special_chars. However, Loofah.fragment.text does not remove the contents of the style element, making this PR necessary.

Changes made

I added a helper function to_snippet(intro_block) that does the following:

1. Removes the style element and its contents from the intro block
2. Strips the tags from the intro_block using Loofah.fragment.text
3. Truncates the intro_block

2 and 3 were previously done in the view, but I moved them to this helper function to consolidate everything into one place, as this sequence is repeated in multiple places in the Find a Project view.