This commits reworks how file caching is performed in Hugo. Now there is only one way, and it can be configured.
This is the default configuration:
```toml
[caches]
[caches.getjson]
dir = ":cacheDir"
maxAge = -1
[caches.getcsv]
dir = ":cacheDir"
maxAge = -1
[caches.images]
dir = ":resourceDir/_gen"
maxAge = -1
[caches.assets]
dir = ":resourceDir/_gen"
maxAge = -1
```
You can override any of these cache setting in your own `config.toml`.
The placeholders explained:
`:cacheDir`: This is the value of the `cacheDir` config option if set (can also be set via OS env variable `HUGO_CACHEDIR`). It will fall back to `/opt/build/cache/hugo_cache/` on Netlify, or a `hugo_cache` directory below the OS temp dir for the others.
`:resourceDir`: This is the value of the `resourceDir` config option.
`maxAge` is the time in seconds before a cache entry will be evicted, -1 means forever and 0 effectively turns that particular cache off.
This means that if you run your builds on Netlify, all caches configured with `:cacheDir` will be saved and restored on the next build. For other CI vendors, please read their documentation. For an CircleCI example, see https://github.com/bep/hugo-sass-test/blob/6c3960a8f4b90e8938228688bc49bdcdd6b2d99e/.circleci/config.ymlFixes#5404
* Move Gopher SVG from partial to image
In minification, Gopher gets wonked. This may also help build times with minification.
fixes#604
* Add margin around external link icon
In minification the links got pushed up against the nav item.
ref #604
* Move Hugo logo in footer/hero to image from partial
This may help with build-time when using minification.
* Move gopher image from site to theme static folder
I put this in the wrong directory in commit: 0e2f2f8f33588f826f26c3b3a581f1014a3d41ac
* Add alt tag copy to Gopher image
When run under gccgo, the test looks for the name that gccgo gives to
a thunk method. This name is not normally visible, but can be seen
when using reflect.FuncForPC as this code does. That name changed in
https://golang.org/cl/89555. Change the test to work with both the
old name "$thunk0" and the new name "thunk0".
Fixesgolang/go#28669
This commit also pulls down the log level for a set of WARN statements to INFO. There should be no ERRORs or WARNINGs in a regular Hugo build. That is the story about the Boy Who Cried Wolf.
Since the WARN log is now more visible, this commit also improves on some of them, most notable the "layout not found", which now would look something like this:
```bash
WARN 2018/11/02 09:02:18 Found no layout for "home", language "en", output format "CSS": create a template below /layouts with one of these filenames: index.en.css.css, home.en.css.css, list.en.css.css, index.css.css, home.css.css, list.css.css, index.en.css, home.en.css, list.en.css, index.css, home.css, list.css, _default/index.en.css.css, _default/home.en.css.css, _default/list.en.css.css, _default/index.css.css, _default/home.css.css, _default/list.css.css, _default/index.en.css, _default/home.en.css, _default/list.en.css, _default/index.css, _default/home.css, _default/list.css
```
Fixes#5203
Bjørn Erik Pedersen