Search feature doesn't work on all docs pages

[cabhishek]

Ways to reproduce:

1. Goto https://nim-lang.org/docs/tut1.html
2. Type json in the search bar and click Module Json from the search result (works)
3. On the actual Json module page type macros (or anything), the search bar doesn't work anymore

This used to work prior to v0.18.0 and is quite critical for discoverability.

Browser shows following error:

dochack.js:1448 Uncaught TypeError: Cannot read property 'toLowerCase' of undefined
    at dosearch_28190 (dochack.js:1448)
    at wrapper_28415 (dochack.js:1527)

Tested on Chrome Version 64.0.3282.186 (Official Build) (64-bit)

Can reproduce on Firefox 58, but error is a bit different:

[mjoud]

The id searchInput is duplicated since #6723.

Nim/config/nimdoc.cfg

Lines 112 to 113 in 4164ec4

	<div id="searchInput">
	Search: <input type="text" id="searchInput"

and

Nim/config/nimdoc.cfg

Lines 142 to 143 in 4164ec4

	<div id="searchInput">
	Search: <input type="text" id="searchInput"

[cabhishek]

@mjoud Good point and it leads to invalid value lookup.

In dochack.js

var elem_28417 = document.getElementById("searchInput");
var value_28418 = elem_28417.value; // undefined