Use bootstraps own img-responsive class for responsive graphs

[arjitc]

Old: http://i.imgur.com/ZKHurbZ.png
When img-responsive is used: http://i.imgur.com/R6sTtiV.png
Automatically resizes as well when I resize the window: http://i.imgur.com/G5QxZN7.png

[LibreNMS-CI]

Auto-Deploy finished, Test PR at http://2633.ci.librenms.org or https://2633.ci.librenms.org

[laf]

Does this negate the need for #2537?

[arjitc]

@laf Yup, does not need the $w and $h values either.

[LibreNMS-CI]

Auto-Deploy finished, Test PR at http://2633.ci.librenms.org or https://2633.ci.librenms.org

[LibreNMS-CI]

Auto-Deploy finished, Test PR at http://2633.ci.librenms.org or https://2633.ci.librenms.org

[Alucardfh]

I just tested this PR and it is indeed working very well.

The only downside is that the bootstrap responsive code doesn't upscale image. Which anyway isn't something we want as it would blur graphs. But this means that graphs are still tiny on big resolutions.

So I also tested the changes in the PR on top of the one I have in PR #2537 which looks quite good to me and avoid my ugly workaround to reduce the blur on graphs.

Current master behavior (resolution 2560x1440):

With this PR :

With this PR + PR #2537 ;

[paulgear]

I've tested this along with PR#2537 and it looks perfect on my browser. Big thanks, @arjitc, and 👍 to merge both this & #2537.