replace resizable js with resize css

[vchrisb]

currently a quite outdated js library resizable is used.
CSS has an out of the box solution with resize, which is widely used, e.g. like here in Github
I would like to suggest to replace the js library by using resize instead.

It should be enough to remove the resizable library and usage and add the following to martor.bootstrap.css

.martor-field {
    width: 100%;
    height: 250px;
    min-height: 100px;
    resize: vertical;
    border-right: 1px solid rgb(204, 204, 204);
    border-bottom: 1px solid rgb(204, 204, 204);
}

resize: both does also work, but requires a bit more investigation how to set a max-width and min-width