Set default user agent for external requests

[lukehsiao]

Many servers will return errors (e.g. 400/403) to requests that do not
set a User-Agent header. This results in issues in both the link_checker
and load_data components. With the link_checker these are false positive
dead links. In load_data, remote data fails to be fetched. To mitigate
this issue, this sets a default User-Agent of

$CARGO_PKG_NAME/$CARGO_PKG_VERSION

Note that the root cause of this regression from zola v0.9.0 is that
reqwest 0.10 changed their default behavior and no longer sets a
User-Agent by default:

https://github.com/seanmonstar/reqwest/pull/751

Fixes #950.

IMPORTANT: Please do not create a Pull Request adding a new feature without discussing it first.

The place to discuss new features is the forum: https://zola.discourse.group/
If you want to add a new feature, please open a thread there first in the feature requests section.

Sanity check:

• Have you checked to ensure there aren't other open Pull Requests for the same update/change?

Code changes

(Delete or ignore this section for documentation changes)

• Are you doing the PR on the next branch?

If the change is a new feature or adding to/changing an existing one:

• Have you created/updated the relevant documentation page(s)?

This change is not visible to the end user, and doesn't have corresponding docs.

[Keats]

Thanks!