[WIP] Remove execute bit from non-executable files. Install relevant build check.

A lot of textfiles in the repository have the execute bit set, ranging from `.html` to `.md`, even to `.jpg` files. A small-scale effort to remove execute bits has been made in the past, in #439.

Rogue executable files cause `lintian` to error, as was reported in #495. A couple of patches have been submitted by @gubatron to fix this issue (here: https://github.com/OpenBazaar/OpenBazaar/commit/bdac972be716f04ec24cc1dd9ebc7e05884e397c and here: https://github.com/OpenBazaar/OpenBazaar/commit/5872d0928f486e074a6a2ac49f37dfdbad4fc8b9), by stripping the execute bit on-the-fly while building the Ubuntu/Debian package.

However, there is no real reason to rely on such hacks. The surest and easiest way to fix this is to have the files themselves be non-executable. This PR clears the execute bit and installs a build check to ensure no regressions happen.

So far, I have removed execute permission from the following types of files:
* `.css`
* `.html`
* `.md`
* `.txt`

What other kids of files shouldn't be executable by default? `.js`, `.json`... ? Help me.