i18n: babel_extract_angular should trim whitespaces in AngularJS templates

This is "AngularJS templates" version of bug 1583757.

At now, translators will get source strings with meaningless newlines and whitespaces like below. Zanata (translation check site) checks the number of newlines, so translators need to insert newlines to silent Zanata validations. It is really annoying and meaningless.

For Django templates, Django provides 'trimmed' option to trim whitespaces in extracted messages. It would be nice if we have the similar behavior to Django 'trimmed' option for AngularJS template message extraction. In HTML case, we don't need to care consecutive whitespaces, so we can simply trim whitespaces in AngularJS HTML templates.

#: openstack_dashboard/dashboards/project/static/dashboard/project/containers/create-container-modal.html:40
msgid ""
"A container is a storage compartment for your data and provides a way\n"
" for you to organize your data. You can think of a container as "
"a\n"
" folder in Windows® or a directory in UNIX®. The primary "
"difference\n"
" between a container and these other file system concepts is "
"that\n"
" containers cannot be nested. You can, however, create an "
"unlimited\n"
" number of containers within your account. Data must be stored "
"in a\n"
" container so you must have at least one container defined in "
"your\n"
" account prior to uploading data."
msgstr ""

We would like to have a string like:

#: openstack_dashboard/dashboards/project/static/dashboard/project/containers/create-container-modal.html:40
msgid ""
"A container is a storage compartment for your data and provides a way for"
" you to organize your data. You can think of a container as a folder in "
"Windows® or a directory in UNIX®. The primary difference between a "
"container and these other file system concepts is that containers cannot "
"be nested. You can, however, create an unlimited number of containers "
"within your account. Data must be stored in a container so you must have "
"at least one container defined in your account prior to uploading data."
msgstr ""