Perf Optimisation: Use EEX Template for SVG Rendering

At present we have a 511 Byte [`template.svg`](https://github.com/dwyl/hits-elixir/blob/master/lib/template.svg) file which we are _manually_ [`String.replace`](https://github.com/dwyl/hits-elixir/blob/12e7f9a9f84d9341c022320587da29b88b7f940e/lib/hits.ex#L30)-ing the `{count}` in:

```
String.replace(svg_badge_template(), ~r/{count}/, to_string(count))
```

This does not take advantage of Phoenix's template compilation for faster rendering.

# Todo

+ [ ] refactor template to `badge.svg.eex`
+ [ ] render using `render(conn, "badge.svg", count)`
+ [ ] apply gzip compression for even faster rendering

> _**Note**: rendering is **not** currently a "bottleneck" in the app,
the DB query is orders of magnitude slower than `String.replace`.
This optimisation should be considered `after` ETS is implemented for caching the count._

Start with lib/hits.ex at line 30 and lib/template.svg, then compare the current String.replace path with the proposed badge.svg.eex rendering. Check how the count reaches the SVG response and how gzip would fit after ETS caching. Done means the compiled template renders the same badge and the requested compression is applied without changing the count output.

Written by the indexing model from the issue text.