💫 Create random IDs for SVGs to prevent ID clashes (#2927)

Resolves #2924.

## Description
Fixes problem where multiple visualizations in Jupyter notebooks would have clashing arc IDs, resulting in weirdly positioned arc labels. Generating a random ID prefix so even identical parses won't receive the same IDs for consistency (even if effect of ID clash isn't noticable here.)

### Types of change
bug fix

## Checklist
<!--- Before you submit the PR, go over this checklist and make sure you can
tick off all the boxes. [] -> [x] -->
- [x] I have submitted the spaCy Contributor Agreement.
- [x] I ran the tests, and all new and existing tests passed.
- [x] My changes don't require a change to the documentation, or if they do, I've added all required information.

spacy/displacy/render.py

@@ -1,6 +1,8 @@
 # coding: utf8
 from __future__ import unicode_literals
 
+import random
+
 from .templates import TPL_DEP_SVG, TPL_DEP_WORDS, TPL_DEP_ARCS
 from .templates import TPL_ENT, TPL_ENTS, TPL_FIGURE, TPL_TITLE, TPL_PAGE
 from ..util import minify_html, escape_html
@@ -38,7 +40,10 @@ class DependencyRenderer(object):
         minify (bool): Minify HTML markup.
         RETURNS (unicode): Rendered SVG or HTML markup.
         """
-        rendered = [self.render_svg(i, p['words'], p['arcs'])
+        # Create a random ID prefix to make sure parses don't receive the
+        # same ID, even if they're identical
+        id_prefix = random.randint(0, 999)
+        rendered = [self.render_svg('{}-{}'.format(id_prefix, i), p['words'], p['arcs'])
                     for i, p in enumerate(parsed)]
         if page:
             content = ''.join([TPL_FIGURE.format(content=svg)