Webstart index template (rebased onto dev_5_0) #2210

This is the same as gh-2210 but rebased onto dev_5_0.

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This fixes customization on webstart template as well as allow to set institution front page on top of webclient.

New configurable variables as follow:
 - ```TEMPLATE_DIRS``` - list of locations of the template source files, in search order. 
 - ```INDEX_TEMPLATE``` - custom template for intro page that appear before login (default value: ```webstart/start.html```)
 - ```WEBSTART_TEMPLATE``` - custom template for webstart (default value: ```webstart/index.html```)


To test it first please create test files and place them in ```omeroweb/webstart/templates/webstart/``` set the following: 
```
$OMERO_HOME/bin/omero config set omero.web.webstart_template 'webstart/test1.html'
$OMERO_HOME/bin/omero config set omero.web.index_template 'webstart/test2.html'
```
and check if:
   - ```http://localhost/webstart/``` shows new template
   - ```http://localhost/``` shows new front page

Second, templates can be also defined outside of the omero directory. In order to test it please create local directory, paste test files in and set the following:

```
$OMERO_HOME/bin/omero config set omero.web.template_dirs '["/path/to/templates_dir"]'
$OMERO_HOME/bin/omero config set omero.web.webstart_template 'test1.html'
$OMERO_HOME/bin/omero config set omero.web.index_template 'test2.html'
```

WARNING: Please make sure ```omero.web.login_redirect``` is unset