Replace "named" Mink selectors with "named_exact" Mink selectors

The "named" Mink selector first tries to find an exact match for its
locator and then, if not found, tries to find a partial match. Besides
other harder to track problems (see comment in the commit in which the
"content" locator was removed), this could cause, for example, finding
an action link titled "Favorited" when looking for the action link
titled "Favorite" (that is, one that conveys the opposite state to the
one found).

Although currently all the acceptance tests are compatible with both the
"named" and the "named_exact" Mink selectors the predefined locators are
modified to use the "named_exact" Mink selector to make them more
future-proof; the "named" Mink selector can still be used if needed
through the "customSelector" method in the builder object.

Signed-off-by: Daniel Calviño Sánchez <danxuliu@gmail.com>