Replace uses of client.get and client.post

We have a `client_request` fixture which does a bunch of useful stuff
like:
- checking the status code of the response
- returning a `BeautifulSoup` object

Lots of our tests still use an older fixture called `client`. This is
not as good because it:
- returns a raw `Response` object
- doesn’t do the additional checks
- means our tests contain a lot of repetetive boilerplate like `page = BeautifulSoup(response.data.decode('utf-8'), 'html.parser')`

This commit converts all the tests which had a `client.get(…)` or
`client.post(…)` statement to use their equivalents on `client_request`
instead.

Subsequent commits will remove uses of `client` in other tests, but
doing it this way means the work can be broken up into more manageable
chunks.

app/templates/views/cancelled-invitation.html

@@ -1,5 +1,5 @@
 {% extends "withoutnav_template.html" %}
-{% block per_page_title %}Invitation has been cancelled{% endblock %}
+{% block per_page_title %}The invitation you were sent has been cancelled{% endblock %}
 {% block maincolumn_content %}
     <div class="govuk-grid-row">
       <div class="govuk-grid-column-two-thirds">

app/templates/views/forgot-password.html

@@ -3,7 +3,7 @@
 {% from "components/form.html" import form_wrapper %}
 
 {% block per_page_title %}
-Create a new password
+  Forgotten your password?
 {% endblock %}
 
 {% block maincolumn_content %}

app/templates/views/two-factor-sms.html

@@ -3,7 +3,7 @@
 {% from "components/form.html" import form_wrapper %}
 
 {% block per_page_title %}
-  Text verification
+  Check your phone
 {% endblock %}
 
 {% block maincolumn_content %}

app/templates/views/verification-not-received.html

@@ -2,7 +2,7 @@
 {% from "components/button/macro.njk" import govukButton %}
 
 {% block per_page_title %}
-  Resend verification code
+  Resend security code
 {% endblock %}
 
 {% block maincolumn_content %}