Add configuration variables for vendor libraries location

[roipoussiere]

Currently, all third-party libraries are provided via CDNs, defined directly in html code.

However there are some use-cases where a developer would want to specify a library location himself:

• to use self-hosted libraries, if he don't want to depend on CDNs;
• to use the same file of a current basecode, so flask-blogging will use cache instead of downloading the same library twice;
• to use an other library version (such as bootstrap4, etc.);
• to update the library to the latest release;
• to fix the website if a CDN is not available (temporary or definitely);

For all of these use-cases, it is easier to use configuration variables instead of monkey-patching flask-blogging.

That's why this pull-request put all third-party libraries locations in configuration variables. This includes:

• VENDOR_BOOTSTRAP_CSS;
• VENDOR_BOOTSTRAP_JS;
• VENDOR_BOOTSTRAP_MARKDOWN_CSS;
• VENDOR_BOOTSTRAP_MARKDOWN_JS;
• VENDOR_MARKED_JS;
• VENDOR_JQUERY_JS;
• VENDOR_MATHJAX_JS;

For instance, a developer will be able to define in main.py:

app.config["VENDOR_BOOTSTRAP_CSS"] = '/static/css/bootstrap.min.css'
app.config["VENDOR_BOOTSTRAP_JS"] = '/static/js/bootstrap.min.js'

Not that by default, the CDNs are used, so it doesn't change the current behavior.

Other minor modifications:

• update some urls in order to use https;
• add documentation about flask-fileUpload configuration variables.

[gouthambs]

@roipoussiere I expect users to override and create their own templates. I am afraid this PR is going to add a whole lot of config variables that most people won't use...

[roipoussiere]

Yes but all of these config variables are optional, so that changes nothing for these most people who won't use them.