ZCS-12939: added context path for images

[k-kato]

Adding context path to images in change password pages.

When zimbraMailURL is changed from / to /test/path, for example,
(A) zimbraPasswordMustChange TRUE

1. Set zimbraPasswordMustChange to TRUE on an account
2. Go to login page of Classic UI (i.e. login.jsp)
3. Log in to the account
4. change password page is shown
5. a path of each image should be /test/path/img/xxx

(B) standard change password page on Classic UI

1. Login to Classic UI
2. click Change Password
3. change password page is shown in a separate window
4. a path of each image should be /test/path/img/xxx

Note:
The target (base) branch is ZCS-12907. #793 is adding the following code to login.jsp.

<c:if test="${pageContext.request.contextPath ne '/'}">
    <c:set var="basePath" value="${pageContext.request.contextPath}"/>
</c:if>