`setDescription` and `setDescriptionI` present a similar API to
`setTitle` and `setTitleI`, but unlike those functions the description
functions are not idempotent - so calling them multiple times inserts
multiple `<meta/>` tags in HTML `<head/>`.
This adds explicitly idempotent versions of those functions which are
handled in a similar way to the title, so that calling them multiple
times has the effect of taking the final value specified.
Because the non-idempotent behaviour of setDescription is not obvious,
this also adds warnings for that behaviour to make it clear what the
effect of multiple calls will be. Unfortunately, setDescriptionIdemp
can't be made a drop-in replacement because developers may have defined
their own layouts which need to take pageDescription into account.
* Add functions for setting description and OG meta
It's common that a website author will want to add a specific
description and Open Graph image to a given page in their website. These
functions are simple conveniences to add these meta tags to the document
head.
I decided against simply adding all possible meta tags, because not all
of them are useful, and even in the case of Open Graph tags, many of
them should be set only once and they should use the same value for the
entire website. In those cases, it's probably better for the website
author to add those tags in their layout template.
Closes https://github.com/yesodweb/yesod/issues/1659
